Ring gauge is the thickness of a cigar. A 50 ring gauge cigar is 50/64 of an inch across. A lancero is usually in the 38–40 range, a robusto is often about 50, and a thick toro can be 52–56 or more. The number is a measurement, not a measure of quality.
The diameter affects how a blend tastes. A larger ring gauge means more filler compared with wrapper, a different burn rate, and more room to place ligero inside the bunch. It does not add flavor by itself. A thin cigar can be full-bodied, and a thick one can be mild.
Length is a separate measurement. A vitola is the combination of length and ring gauge, often with a traditional name such as corona, churchill, or perfecto.
Full chart and size guides: ring gauge and length cheat-sheet.