What is the gauge of string?

The gauge is the thickness of a string you’re using, 15 gauge which is the heaviest or thick like a rope, then 16 gauge which is common in most that most people use that gauge or thickness. Then 17 gauge which is the thinner with high playability and now you could get an 18 gauge which is even thinner. the problem with all of these gauges of string is durability versus playability.