In any post there are three attributes:
What is the meaning of the active in a post?
"Active" shows when the post was last active. Active means that it has a change: an edit, an answer, an edit to an answer, bumped by Community♦ etc...
It might happen that the "last activity indicator" displays an activity which is nowhere to be seen. This can be the result of:
modified by whoeverCastedLastReopenVote);
New comments don't make a question "active".