Do you know what this part is and where I can buy it

From what i can tell, it is part of a union. Possibly a screw in type depending on pump model. I dont know if pentair has proprietary unions or not, but im sure others who know more will help you more specifically

Agree, it's a union and your finger is point to half of it. There are three pieces your fingertip is touching one part, your first knuckle is touching the second of three and the third part is on the other side of the tightening nut your first knuckle is touching. I would say find the size of it on the molding of the union and you will most likely find one at the hardware store. To me it would appear to be a 2". If it's leaking from the threads Might try to take it out if it's threaded and not cracked, then apply teflon tape and reinstall.

Looks like a pretty standard 2" (could possibly be 1.5") union. Unscrew it and take it to home depot/lowes. You may need to replace both sides as some are a little different.