It's used for almost all questions in meta, and seems a bit useless or misused.
What is it for? Should the previous questions have their tags cleaned accordingly?
It's pretty much explained in the tagwiki. It's a standard tag on all meta sites:
The question you're asking is designed to solicit opinions or best-practices on a particular topic, with the goal of reaching community consensus.
Basically there is a small subset of tags, one or more of which must be included in a meta question. The others are things like bug reports, feature requests, and asking for support from the people that run the system.
So "discussion" is just the one that covers "everything else". It means we're just chatting about some aspect of the site or other rather than asking for direct/immediate action/attention.
It is a little bit silly, but for some reason which I do not know, all questions on meta have to be labeled as at least one of bug, support, feature-request, and discussion (unless it is posted by a moderator). I have been trying to use one of the first three whenever possible because, as hippietrail explained, discussion effectively means “this is not a bug report, a support request, or a feature request.”