A troll is someone who winds up other posters in various ways. You can google it and it will tell you; MN trolls are not really any different.
They might be:
a) malicious
b) attention-seeking
c) joking around
The last group are tolerated fairly well, especially if it's a good wind up. The former two are not.
We have some "famous" trolls - the one I remember best (because I was involved in it) is CVQ, a pretty disturbed class b) troll, who upset several people. She claimed to have had a baby through rape and was constantly attention-seeking - anyway, she upset one regular poster enormously and it all went tits up from there.
Another one, I think it was The Wilted Rose, managed to get money out of posters for something.
More recently was Dizzymare, who was a class a) and b) troll - she claimed to have been pg with twins and then lost them at 22w, I think. Lots of heartache all round until she was outed as a regular troll on several forums, not just MN.
The point in general is that if something feels unbelievable, people may start calling 'troll'.
Ways to avoid trollery - don't namechange unless you absolutely have to; don't do an "AIBU by stealth" - i.e. dripfeed information that substantially affects your OP; and get your facts right. Any slip will result in accusations of trollery!
Good luck!