Plain English Meaning
A nationwide system of weather radars that not only shows where rain and storms are, but also shows which way the wind inside them is moving. It is the radar that produces most of the weather images you see in briefings and on aviation weather websites.
Full Definition
A network of ground-based Doppler weather radars operated by the National Weather Service, the FAA, and the Department of Defense that detects precipitation, measures its intensity, and tracks the motion of air within storms. The 'D' indicates Doppler capability, which allows the radar to measure how fast precipitation particles are moving toward or away from the antenna, revealing wind shifts, rotation, and storm structure that older radars could not see.