what does " forumdisplay.php?f=92" do?

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When I place my cursor to a site, the address display from browser shows something like:

hhtp://www.vbulletin.org/forum/forumdisplay.php?f=92

how does the "f" and "92" passed to the forumdisplay.php?
I know "f" and "92" are not php variables since they don't have $ sign in front of it. Where can I read something to understand the internals and be able to trace the forumdisplay.php program (i.e. what happens after the "92" assigned to "f", which part of the php received the
value 92 etc).

This may look like a simple question, but I could not find an answer. Thank in advance for answering.