Proof is an all but meaningless expression of alcohol strength.
The origins of this measurement date to the 18th century. Alcohol was determined to be of proof when a solution of the alcohol with water could be poured on a pinch of gunpowder and the wet powder could still ignite. It is no longer legally required on labels of spirits but is almost always found there.
What is required on the label of spirits sold in the USA is the alcohol by volume or ABV. ABV is the standard measure of alcohol in an alcohol beverage expressed by a percentage of total volume.
Further than being irrelevant today, proof is calculated differently in the US and the UK, which of course creates much confusion.
The US method of calculating proof is to simply double the Spirit’s alcohol by volume (ABV). A pretty simple albeit useless calculation that can be accomplished by anyone with a grade 4 math education.
The UK system calculates with a more complicated ratio of 7:4. A Spirit with an ABV of 40% would have a US proof of 80, whereas in the UK, it would have a calculated proof of 70.
Don’t worry, there will not be a test.