That's the meaning of SPAM. Basically those posts that are devoid of meaning or a point to exist. In general extremely short messages can be considered spam. For example just quoting someone else and saying "I agree" or saying nothing to signal that is what you think feel for many like a waste. Another example of spam is long posts that are actually completely unrelated to the topic at hand, in a different language or trying to advertise something external.
So not all short posts are spam and not all long rants are valid.
In my opinion to decide about SPAM post it is all about communication. Does the post allow for a discussion to flow? Is it trying to bring something to the topic at hand or is it creating a new topic we can chime in.
What about you?