What is Uncompressed Sim Textures?

It's been there from the start. It makes the skin texture super smooth but this will make your comp work a bit harder. Some of the skin textures look really bad with uncompressed texture off and that's why I turned it on.

If you leave it unticked the game uses a compression algorithm to decrease the amount of memory the textures use. Zoom in close and you can see some artifacts as a result of the compression.

If you have a lot of VRAM, make sure you enable it because, as others have said, the textures look much smoother.

It's been there since the beginning. I have mine checked. My sims look better to me with them check. I have no idea the technical side of it. I just know what I see, and my sims look more crisp with it checked.

Yes, it's been there from the beginning and will make your Sims' skin look smoother, less blotchy, features will be more defined, etc. Be advised though, if you don't have a gaming rig or a very good graphics card, it can cause a lot of lag in your game. Probably because it'll uncompress ALL Sims in your save, not just your own. At least that's what I read about the feature in another thread somewhere a long time ago.

Wouldn't hurt to try it and see what happens, might be just fine in your game and you'll end up with more beautiful Sims. :)