Sometimes the most interesting parts of a piece of media are the parts of the story that aren't the main plot. Side characters can often be the ones that stick with fans the most (as they have the most freedom to be quirky or more morally ambiguous than the protagonist), and the fan-favorite characters are typically the secondary ones. The Harry Potter series in particular has a real wealth of side characters, as the Wizarding World is quite expansive. The fandom around the books has really latched on to some of these characters, with some of them getting more attention these days than the Golden Trio does.
The Marauders are some of the most beloved characters from the Harry Potter series, despite the very limited time that is actually spent with them over the course of the series. Their personalities just shone through, and have grown beyond the canon of what's actually on the page, as many fans have expanded on their characters in ways that are just universally accepted by the fandom as their own canon (or fanon, in other words). But what is it about these characters in particular that is so attractive to fans, and how did they manage to create a whole subsection of the fandom that is solely dedicated to these side players?
Who Are The Marauders?
The Marauders are first introduced in the series through the Marauder's Map, which is a magical artifact that comes into Harry's possession after Fred and George Weasley steal it from Filch's office. The Map has the ability to show where anyone is at Hogwarts and tracks their movements around the property. It can only be opened and viewed using a particular incantation ("I solemnly swear I am up to no good"), and the Map can even talk back, as is evident when Snape attempts to open the Map without the password and is insulted by the makers. But who were the makers, exactly? It turns out that the minds behind the Marauder's Map have much more of a connection to Harry than he thought. The Marauders were a group of students that went to Hogwarts in the 1970s, comprised of Sirius Black, Remus Lupin, Peter Pettigrew, and James Potter.
Harry does eventually encounter most of them over the course of The Prisoner of Azkaban, some more favorably than others. James, of course, is his father, who died fighting Voldemort when Harry was a baby. Remus Lupin was Harry's Defence Against the Dark Arts Teacher in his third year, Sirius Black is an escaped convict from Azkaban that Harry eventually finds out is his godfather, and Peter Pettigrew, was found to be hiding in his Animagus form as Ron's pet rat Scabbers for years, only revealed during the climax of Prisoner of Azkaban.
While they were in school, the Marauders, as they called themselves, were a group of friends in Gryffindor that had a penchant for pranks and rule-breaking. They were talented wizards, as the creation of the Marauder's Map was a complex piece of magic, and they used it for their own mischief-making, as well as inadvertently helping future mischief-makers at Hogwarts with their goals. Harry himself is deeply tied to the group as his father James was a part of it, and his mother - Lily Evans - was sort of an honorary member of the friend group when she began dating James.
Why Are The Marauders Important to Harry Potter?
The Marauders are actually really important to the plot of the books, though the movies tend to skip over some of the details surrounding them. The Prisoner of Azkaban is obviously where they take center stage, but their story continues to be important throughout the series. One of the plot points that is carried through the books is Professor Snape's hatred for Harry, which stems from the way James treated Snape while they were in school. Snape was disliked by the Marauders, and they (mainly James and Sirius) were intent on teasing and humiliating him whenever they could. To add insult to injury, Snape was in love with Lily, but she only had eyes for James (though perhaps it's not surprising that Lily didn't reciprocate Snape's feelings after he called her a Mudblood).
Sirius Black was supposed to be the Secret Keeper for James and Lily to hide them and Harry from Voldemort during the war, but he was sent to Azkaban after Voldemort found the Potters and killed James and Lily (and attempted to kill Harry and failed). As it turns out, he wasn't actually the Secret Keeper at all, and it was instead Peter Pettigrew, who was the one who ratted them out to the Dark Lord. This is, obviously, the plot of The Prisoner of Azkaban, but it's the other main contribution that the Marauders as a group have to the overall storyline. The individual characters play their parts as well, but this is kind of the extent of the importance of the group of boys as a whole, as far as the plot goes.
That being said, there is a large portion of the Harry Potter fandom that is especially interested in these characters and this time period within Hogwarts, and it really had a resurgence during the pandemic, when online discourse grew as people weren't able to leave their houses as much - though this subsect of the Harry Potter fandom has always been quite active. The fans who love the Marauders era have even developed certain characters that were mentioned once as classmates of the Marauders and given them personalities and backstories that are widely accepted by pretty much everyone in the fandom. It's a fascinating example of the material outgrowing even what the author intended, as the dedicated fans have pretty much made it their own. It just goes to show that even side characters can be incredibly memorable, and sometimes even the most beloved characters in the whole series.