@May_Otterview sorry for the late reply but for the most part the descriptions of what spam is were on point. Spam is usually the continual posting of things that don't usually add to a conversation or the continual posting of things that disrupt conversations.
So in terms of online it's like if you were in-game talking to @Nora_Wrenchfizz1 and I came along and just kept saying "Hi how is everyone"
"Hi how is everyone"
"Hi how is everyone"
"Hi how is everyone"
"Hi how is everyone"
Specifically, when it comes to forums It could mean something like these as replies to an idea (all by the same person of course)
"wow that is cool"
"How did you do that"
"Where can I do that?"
"When will I find that furry thing Lukas owns?"
So the point is really, try and condense your posts into one post rather than 5 
Hopefully, that makes sense!