MatCaps (for Material Capture) are a simple and easy way to make your objects look nice when you’re working on them, for example while sculpting. They are spherical images that are used to add a material, but also lighting and reflections to your 3D objects.

It makes it more pleasant to look at for your eyes when sculpting, and it can also help identify areas that need more work in your model, for example to find lumps on a surface that need to be smoothed out. It can even be an easy, cheap way of rendering a presentation of a sculpt without the need for setting up lighting, as this is all baked into the MatCaps.

Did you know that you can add your own matcaps to Blender? And there are many interesting free ones to be found online.