Each degree of alcohol proof is equal to 0.5 % Alcohol By Volume (ABV). As an example, 100 proof is the same as 50% alcohol by volume.
"Proof" is a historical term that was used to describe how much alcohol was in a batch of spirits. Gunpowder was added to a barrel of alcohol (vodka, rum, etc.), and if the alcohol would ignite then it contained at least 57.15% ABV. If it did not ignite, it was "under proof." Now, percentage or ABV is the commonly used term and measurement of alcohol.