What is being toxic?

So i got a game chat restriction after 1 game someone was flaming me so all i said was fuck off, and another game i admit was toxic and i called someone a r%%%%% once the entire game. In both these games this was the only thing i had said (can post chat logs if needed). So im wondering so that i can become a more positive player what can i say? in the sense that am i not allowed to swear/say something negative about anyone at all or are there certain flag words i shouldnt say.