Beta-alanine is a modified version of the amino acid Alanine. Once ingested, it turns into the molecule carnosine which acts as a potent acid buffer in the body.
Increasing body stores of carnosine can both protect against diet induced drops in pH (such as occurs from ketone production in ketosis) and from exercise induced lactic acid production.
By buffering lactic acid, users are often able to complete more reps before exhaustion allowing muscle size and strength gains.
Beta Alanine may cause a skin "tingle" shortly after consuming. This is harmless and normal.