I was tightening one of the bolts under the hood for the passenger side strut and I slipped off the wrench and punched this thing and broke it. When it broke it let out a hissing noise like it was pressurized or something. Looks cheap but I don't know what the heck to ask for at the store, or what it even does.

Looks like some sorta valve. Its located right by the PCM. 95 Lt1 Trans am if that makes a difference

 

HVAC vacuum check valve. That routes vacuum to the HVAC control panel in the dash, and connects to the vacuum reservoir in the passenger front fender. Should be a fairly cheap part.

Shoebox always has the answer:

http://shbox.com/1/vac_check_valve.jpg

 

One good thing that you found out is that your vacuum lines and reserve tank are holding up. It would not have hissed if they weren't.

A new check valve may not look exactly like the old one, but it will work the same.