The term SPAM is hard to define, but in short, is double posting (or more) usually completely random gibberish that is meant either to bump the thread to the top of the list, OR for no reason at all other than to attract attention.
that is spamming at it's worst but spamming can also be short sentences that don't add anything to the thread eg. "looks nice will dl" this is gennerally a bad thing and is frowwned upon by the wider community
Or saying something like "ok" "cool" "awesome" enough times will get you benned because of unuseful posts(spam) SPAM SOI SOI SOI SOI SOI SOI SOI SOI SOI SOI SOI SOI ROFLCOPTER