depends on the circumstances and extent of the cut. For small cuts, a folded dollar bill works fine. If you're far from home, but can get to a store, things like duck tape, or cloth surgical tap can save larger cuts.

Many years ago on a tour, I taped together a 700c tire that was cut almost in half. I replaced the tube, pumped it enough to get shape, and taped tire and rim with surgical tape for about three inches to either side. I had to remove the brake shoes for safety, but it survived over 50 miles until I could replace it.