REAL ID is a federal law, not an actual piece of ID. Congress passed the REAL ID Act in 2005. The act established minimum security standards for state-issued driver licenses and ID cards. Starting October 1, 2021 anyone traveling by airplane or visiting certain federal facilities must use a REAL ID compliant license or ID card.
You can use any of these identification options to board domestic flights:
See the full list of ID documents accepted for air travel.