A necropost is a post in a post on an "old, abandoned thread that has been considered 'dead' for a while...that no longer serves any purpose but is bumped back up to the top of the forums by someone posting in it."
If the topic is old enough and the post doesn't add anything to the topic, then a post qualifies as a necropost. However, a post in a sticky or ITopic is not a necropost if it adds more useful information. Necroposting on purpose is not a good idea, as they are unhelpful and clog up the top of the forum with useless and unneeded topics.
If the topic is not dead, but the post contributes nothing to the discussion, then the post simply counts as spam, which is against the Community Guidelines.
There is no official amount of time that turns a post into a necropost, though when describing a necropost Scratchers may say that bumping a thread after a month or so of inactivity would count as necroposting. However, if the topic is important enough or the post adds more useful information to the topic, then it is not considered a necropost.
Necroposts sometimes happen by accident, where a user posts on a topic without checking the dates of the previous posts.
New Scratchers will often necropost without realizing what they are doing. This usually happens by accident because the user does not realize that the topic has not been posted on in a long time or otherwise does not know that necroposting is generally frowned upon.