The contract is for three missions specifically: the NROL-87, NROL-85, and AFSPC-44 launches.
NROL stands for National Reconnaissance Office Launch, and AFSPC stands for Air Force Space Command. These missions are generally classified, so it's difficult to tell precisely what kind of work they'll entail, but most likely they'll involve sending satellites into orbit for information purposes.
According to the Defense Department, SpaceX will be responsible for "launch vehicle production, mission integration, mission launch operations/spaceflight worthiness and mission unique activities for each mission". So practically everything involved in the mission.
The AFSPC-44 mission is expected to be completed by February 2021, where as NROL-87 and NROL-85 is scheduled to be completed by December 2021. According to the Defense Department's release, two companies competed for the contract, but SpaceX was selected.
Colorado-based United Launch Services was awarded the remaining $441 million to send up three missions of its own: SILENTBARKER, SBIRS GEO-5 and SBIRS GEO-6.
"SpaceX is proud that the Air Force has chosen our company to support our country's defense with these critical national security space launches," said SpaceX CEO Gwynne Shotwell, "and to continue providing the best value in launch with the proven Falcon 9 and Falcon Heavy vehicles."
SpaceX has been awarded a total of nine national security space launch missions and has, to date, completed 66 successful missions.