Plain English: A digital tower shows controllers a live, reconstructed picture of an airport on screens, built from high-definition cameras and sensors, instead of an out-the-window view from a physical tower cab.
Technical: "Digital tower" emphasizes the technology stack, the camera array, pan-tilt-zoom optics, infrared and thermal sensors, and the software that reconstructs the airfield with overlays, regardless of whether the controller is at the airport or at a remote centre. It overlaps heavily with "remote tower"; the terms are often interchangeable.
See remote tower vs digital tower for the distinction, and the map for where digital towers are deployed.