Plain English Meaning
The airspace around a smaller airport that has a control tower. You must talk to the tower on the radio before flying in, but you don't need permission from radar controllers like you would at a busier airport.
Full Definition
Controlled airspace surrounding an airport that has an operating control tower but does not have radar approach control or a high volume of instrument traffic. Class D typically extends from the surface up to 2,500 feet above the airport elevation, within a radius of approximately four nautical miles. Two-way radio communication with the control tower must be before entering, and the airspace reverts to or G when the tower is closed.