All seven Star Trek: The Next Generation cast members reunite for ten episodes being treated as the fifth TNG movie to give the crew the "Undiscovered Country" send off they never got thanks to the wet fart known as Star Trek: Nemesis.
Since you last saw them:
Picard died of Irumodic Syndrome and a long-lost Soong gave him a synthetic body.
Riker & Troi left the Titan and retired on a remote planet with their daughter.
Worf is still a member of the House of Martok, but is the most affected by the Dominion War of the crew.
Geordi remains in Starfleet and his daughters from All Good Things... appear in the series.
Data is still dead. Spiner is playing Lore. This is the first time since Descent, Part 2 in 1993 that Lore has been acknowledged (outside of a gag in Lower Decks). He was ignored in both Nemesis and Picard S1 because the writers of both never watched TNG.
Beverly has left Starfleet and is operating like Doctors Without Borders travelling around providing medical help to needy planets. The series begins with her and a SPOILER character finding something that kicks off the story.