Basically, not all states allow online poker to be played within their borders. However, due to US federal law (The Wire Act) that prohibits moving funds for gambling of any type over state lines, all the players in a poker game must be within the state.
Currently the only legal markets are in Nevada, Delaware, New Jersey, Pennsylvania, West Virginia, and Michigan. Online poker went live in Michigan on January 29, 2024. West Virginia has passed legislation authorizing online poker but no sites are live there yet.