Plain English Meaning
Towered airports have a control tower telling pilots what to do. Nontowered airports don't, so pilots talk to each other on the radio and follow standard procedures to stay out of each other's way.
Full Definition
Two categories of airports distinguished by whether an operating air traffic control tower is present. A towered airport has an active control tower whose controllers issue clearances and instructions that pilots are required to follow within the tower's airspace. A nontowered airport has no operating control tower; pilots coordinate with each other directly using standard procedures and self-announced position reports on a common radio frequency.