What is this plumbing part?

I have a part that is leaking, and I need to replace it, but I have no idea what it is or does. It is right inside he main shutoff, and it makes a hissing sound when water goes through it. Below is a picture. If someone could give me a quick description that would be great!

That is a pressure reducing valve. It looks like it is leaking through the union, which means it is just a bad gasket. You should be able to un-screw that union and take the gasket out and replace it with a new one.

Thanks.

A few more questions about this, Ive also been having an issue with the water pressure in my house where I shut the water main off, and the pressure doubles for about a week then goes back to normal. Its happened a few times. Lately, there have been times where I turn a faucet on, and we have no pressure for about a minute, then all of the sudden I hear the hiss, and the water pressure comes back to normal.

Does it sound like this pressure valve is the culprit? Do you think this could all be caused by the bad gasket, or does it sound like the whole thing is bad?

If so, are these things easy to replace, or should I leave it to a pro. I have a little plumbing experience, but i'm definitely no pro.

These things need rebuilding or replacement every ten years or so.

It sounds like you are due.

Prv

You are describing the symptoms of a bad PRV. The degree of difficulty for changing it will depend on many factors not apparent in your photo. It coudl be a simple 10 minute job, or one that takes an hour or more using several additional fittings, partly dependent upon which valve you buy to replace it.

Thanks everyone for the help!

I have one more follow up question. I can buy the same exact model as a replacement. If I do that, it looks like I just have to shut the water off, unscrew the top and bottom fittings, and screw the new one in. Am I missing anything? Also, I dont have a pressure test tool, is it ok to install with the factory setting of 50 psi, and assume that everything is fine, assuming the water pressure feels fine?

No...you need to set the PRV to the pressure you like / want and then take a reading with a pressure gauge and set your Expansion tanks air pressure to match it...personaly I like to set the tanks air pressure a few #s less than the water pressure...

He never said he even has an expansion tank?

I can't read the label, but it looks like a Wilkins 600. In any event, if you purchase the same brand and model as yours, it will be a relatively easy swap out.

You would be OK with the factory setting. But a pressure gauge costs about $10 at the hardware store, so you may want to check it anyway.