After UVC, which does not reach the surface of the earth, UVB is the most common type of UV ray that is in the sun. UVB is absorbed by the skin and manifests itself by redness. So by monitoring how red you get you can get an idea of how much UVB you're getting.
UVB is well-known to cause skin cancer, specifically non-melanoma skin cancers over time. It is thought that it's not just one session of UVB that causes skin cancer, but the cumulative dose over the course of a lifetime. That's why most BCCs and SCCs happen on sun-exposed areas such as the face and ears. Most sunscreens over the counter that have SPF are designed to protect you from UVB. The higher the SPF, the more protection you have.
It's important to read the actual ingredient, though, as mechanical blockers, such as zinc and titanium, have better UVB protection than other types, like avobenzone.