$block codes can be used to have content dynamically display something that is specific to the user. For example, "$block[username]" in a post will show as "YourUserNameHere". Here is a listing of all of the available codes:

User Name: $block[username] Site: $block[sitename] Access Level Name: $block[userlevelname] Access Level #: $block[userlevelnumber] Participation: $block[userparticipation] Handle: $block[userhandle] Raid Points: $block[userraidpoints] Title: $block[usertitle]