ATH stands for an all-time high, which is a term used to describe a moment when e.g. a cryptocurrency or a stock breaks all its past records and is traded at the highest price that it has ever reached.

For example: Bitcoin broke all its past records and is now traded at a new ATH, $20 000 per coin.

Compound interest is the 8th wonder of the world.

Albert Einstein
Mathematician and physicist, Winner of Nobel Prize in Physics