it means one that meets the criteria of the real id law. it's has extra scrutiny so you are who you say you are, are legally here, etc. law went into effect in 2005, naturally the feds did not give the states a red cent to implement it, and they've had to extend the compliance date for over a decade at this point.

it's supposed to end up that you can't use your license as the id you need to show upon entering a federal building or getting thru tsa security at the airport. if you have a passport/passport card, those will always be acceptable id