<Only applicable if the text "first post" (Or similar spellings) is actually in the first post of any given forum, topic, video comments section etc.>
"First post" is most often used to describe ones "ownage" over all other posters that plan on making a comment.
I.E. "I was here first" "I am better than you" etc.
Its childish to most but to those who would hopefully be "First poster", seeing your "first post" stolen can mean a tragic loss emotionally, often accompanied by feelings of hopelessness, failure, despair and in extreme cases, suicide.
omg first post you all fail.
Wow, nice topic blah blah blah... FIRST POST!
Technically I just got "first post" on the "first post" definition >.>