A necropost is any post on a question that is old that doesn't add any new, useful, or relevant information. For example, if you were to post "turtles are my fave" or something else similar on a three year old question about, for example, the locations of all of the fossils in each game.
To avoid necroposting, just try to not post irrelevant information on questions older than a few months. Only post on old questions and answers if you know for a fact that you are adding both new and relevant information.