From Wikipedia:

A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography and the production of integrated circuits (ICs or “chips”) in particular. Masks are used to produce a pattern on a substrate, normally a thin slice of silicon known as a wafer in the case of chip manufacturing. Several masks are used in turn, each one reproducing a layer of the completed design, and together they are known as a mask set.

In the case of the Skywater PDK, we have 33 different layers and so 33 different masks are needed. The masks have to be extremely precisely made, which is why the whole set of masks ends up being so expensive. You can see a list of all the layers and what they are used for on this spreadsheet.

For experimentation, a Multi Project Wafer is often used to lower the costs.

The process of using the masks to create the patterns on the silicon wafer is called photolithography.