Basics
Proof
Proof is a measure of how much alcohol a drink contains. In the US, proof is twice the ABV, so an 80-proof spirit is 40% alcohol by volume.
In practice
Most gin, vodka, whiskey and rum is sold at 80 to 100 proof (40 to 50% ABV). "Overproof" rum is stronger, often 120 proof or more. The UK once used a different proof scale, which is why labels now give ABV instead.
Proof, in short
What does proof mean?
Proof is a measure of how much alcohol a drink contains. In the US, proof is twice the ABV, so an 80-proof spirit is 40% alcohol by volume.