What is this part used for???

I just got a tank setup from a buddy of mine. It is a 110 gallon tank with 1 corner overflow.
I am trying to make sense of all the plumbing.

Can someone please tell me what that clear part is used for. On it, it says something about 75psi back pressure.
It was connected between the main pump and the UV sterilizer.

Attached you will see it.
Thanks ahead of time guys.

 

looks like a check valve

 

is there a ball and spring inside of the clear plastic piece? if so then it should be a check valve or back pressure reg. more commonly known as a one way valve water can flow freely in the direction its setup in but not the opposite way

 

Thank you guys for answering my question.
You always come through

 

Throw it away, you should not need it and it will eventually fail and cause you problems. Many people think they are necessary to prevent a sump from overflowing in a power outage but in reality properly placed returns and the overflow box with a standpipe are all you need. Check valves have no place in most tanks and are a false sense of security.