,

What exactly is
a squawk and what is it's function?


That refers to the use of transponders or "secondary radar" (same
thing). As you may know, radar works by having a ground station send
out a radio signal and listen for the reply. The analysis of that reply
allows the detection of distance and direction of (metal) objects.

Most aircraft in addition have a "transponder" onboard, which reacts to
being hit by a radar beam by actively sending out a code, which is then
received by a secondary radar antenna mounted on top of the primary
radar. Hence, the air traffic controller will not only see the aircraft
on his/her screen, but each aircraft will have a code displayed next to
it. In its simplest form, this code consists of 4 digits which can be
set on the transponder by the pilot at the direction of ATC. This
four-digit-code is called "squawk" for historic reasons. So, ATC would
say "United XX, squawk 4536", and the pilot would then set that code,
which would then appear on the radar screen of the controller.

Having said all that, are you familiar with Wikipedia or Google?

http://en.wikipedia.org/wiki/Squawk_code

--
Thomas Borchert (EDDH)