The payload will be delivered to low Earth orbit, per the mission parameters, but we don't know anything else about its goal, design, or intended mission, because it's all classified.
According to Space.com, the launch will involve a re-entry attempt by the first stage of the Falcon 9 rocket at Cape Canaveral's Landing Zone 1 - SpaceX has pulled off 20 such re-entries previously. Aside from its name, little is known about the payload. (SpaceX has had 17 consecutive successful landings, so they're getting pretty good at this). It wasn't until earlier in January that the company was able to confirm a hard time or date for the launch.
The new nine-engine rocket will launch from pad 39A and will take the secret payload to a low orbit.
SpaceX just sent a secret government mission to space.
As usual, SpaceX will stream the launch of the Falcon 9 mission, and the webcast should begin about 15 minutes before the launch window opens.