Confused... Progesterone levels?

On my first round of Clomid. Went for my day 21 bloodwork today and just wondering what a good progesterone level is for a woman on Clomid?! And not to sound stupid, but what exactly is progesterone and why do they need to check my levels?! Will it determine whether I ovulated or not on the Clomid?! Soooo confused, just want this Clomid to work, been TTC for 2 years now!!