What does an acidic substance release?

Answer 1

Acids and bases have many definitions, but for your question, these will do:

Bases are proton (H+) acceptors.

Acids are proton (H+) releasers.

Answer 2

An acidic substance releases hydrogen ions (H+) when dissolved in water. These hydrogen ions are responsible for giving acidic substances their characteristic sour taste and ability to react with other substances.