Car trouble

[Moviefan1203]

So I had my car's oil changed today, and they come and tell me that I have a major issue with my engine. My car is a 2005 PT Cruiser, and it has about 31,000 miles on it. The cylinder gasket needs to be replaced, now I know very very little about cars, so I thought I would ask here. The Chrysler dealership that is doing the work neglected to inform me, although to be fair I forgot to ask before I left. Luckily for me, most of the cost is covered by warranty, and I only have to pay a $100 deducible. But my questions are:

A) What exactly does this part do?
B) What would have caused it to go bad?

Thank you.