A severely overused buzzword (similar to 'like') which is commonly used by people who are unable to have an actual argument, commonly used in the online community. This is probably used because it's much easier to get people to join your side when you call someone Toxic, rather than saying "X hurts my feelings" or "X is annoying."
Person 1: "That's dumb. I disagree."
Person 2: "RUDE! TOXIC! HATEFUL!"
Person 1: "You clearly can't take cri-"
Person 2: "ADMIN! PLEASE BAN THIS PERSON! HE ATTACKED ME!!"
In the gaming world, someone toxic could be described as a hater, who chats so much shit that you wanna break your headset... Or simply mute them. This applies to PCs and consoles. Toxic gamers don't get along with anyone other than their friends, honestly no one likes them. They also usually put new players down, instead of helping them to get to know the game.
A toxic community is just full of people described above.
"Don't even tell me that PC is superior to consoles..."
"It is, but besides that, console gamers are so toxic. You can't even have fun without someone telling you that you're bad"