"Fanboy" is a term that refers to an overeager and obnoxious fan of a certain franchise who typically ruins it for others.
On its own, there's absolutely nothing wrong with being a fan of a game franchise you enjoy and show your excitement and appreciation of those games, the problem comes with certain fans who take it far too seriously. They tend to be obnoxious, complain constantly, overthink everything, reject other's opinions, act authoritative and entitled, refuse to listen, NEVER shut up about it, feel threatened by people who don't like the games just as much as they do, make inappropriate fan art, etc. Fanboys also tend to blindly defend clearly anti-consumer practices that companies do, allowing those companies to keep getting away with it. Often they'll harass regular fans because they don't agree with their opinions and say the regular fans are "not true fans" sometimes to the point of completely ruining the franchise for them.
Another type of fanboys are the "classists", who are more prevalent in long lasting franchises like Sonic the Hedgehog and Pokémon. These are fanboys who believe anything within the franchise besides the classics they grew up with is automatically garbage, worship the classic games like they were perfect, and refuse to even give the newer games a chance.
Five Nights at Freddy's, Undertale, Minecraft ,Sonic the Hedgehog, and Bendy And The Ink Machine are five notorious examples of fanbases filled with toxic overzealous fanboys and have the notoriety of creating awful fanart sexualizing, ponifing, humanizing and shipping the characters. The Sonic the Hedgehog fanbase is also notorious for creating generic and uninspired original characters on DeviantART.
Another example of overzealous fanboys are the "elitists", who worship the franchise as if it's some kind of god and look down on fans of other similar games. The Legend of Zelda fanbase is infamous for having elitist fanboys. Their worst action was attacking video game critic Jim Sterling for giving Breath of the Wild a 7/10 which caused the Metacritic score to drop from 98 to 97 (they attacked Jim over the game's score dropping ONE point), as well as DDos'ing his website, despite him saying he liked the game. They're also desperately trying to raise the game's user score from 7.4 to the highest score possible in order to bury the negative reviews.
Perhaps the biggest problem with fanboys, is that while they are usually the minority, they tend to be the most vocal and toxic, which in turn makes outsiders think that the whole fanbase is like that and in turn they'll be less likely to give the games a chance. However it should be noted that even if a fanbase is infamous for being full of toxic fanboys, not all of the fans have to be like that. SomecallmeJohnny is a popular example of a reasonable and healthy Sonic fan.