"First Reply" or "First Replying" is the act of roaming forums, seeing a topic that has been newly created, and quickly clicking and typing "First!" or "First reply!" or the most common "FIRST REPLY!".
More than often, others too are attempting to post first, so you will see multiple "first replies".
This act is often discouraged, as it serves of no relevance to the topic creator (or anyone else for that matter).
Person 1: *Creates new topic.*
Person 2: FIRST REPLY!
Moderator: Annoying child, BURN! *Presses ban button."