This account is fairly old but it is only recently that I have started "contributing" to the site; I have seen that term popping up in some places, in the thread about reporting and in the FAQ, but that only makes reference to the poisoners, not about the concept itself, what does that mean exactly? I want to know so I do not do it and I end up getting myself banned.

I see it as malicious tag editing.

Deleting valid tags, change valid tags to "send a message", going against "tag what you see", tag editing wars.