Tick() provides the time in seconds that has passed since 1st of January of 1970. It actually provides quite a few uses.
What it is most used for is counting how many seconds have passed from one point in the script to another, since tick() is always increasing second by second.
An example of its use would be the following:
local current_time = tick()
wait(3)
print(tick()-current_time)
That would print out 3, as 3 seconds passed. You can find more stuff on it here:
Hope this helps! 