Plain English Meaning
NEXRAD is the country's main weather radar network. It bounces radar signals off rain, snow, and storms to show where the weather is, how strong it is, and which way it's moving. The colored radar pictures pilots see in cockpit weather displays and on briefing websites come from this system.
Full Definition
A network of high-resolution Doppler weather radars operated jointly by the National Weather Service, the FAA, and the Department of Defense, which detects precipitation and atmospheric movement and produces the radar weather imagery widely used in aviation weather products. Each site transmits a radar beam, measures the reflected from precipitation, and uses Doppler shift to determine the and direction of that precipitation, allowing the system to , coverage, and movement of weather across the .