Plain English Meaning
A weather balloon carries a small sensor package up through the sky, sending back readings of temperature, humidity, and pressure at every level it passes. Tracking where the balloon drifts also reveals the winds aloft. The result is a top-to-bottom snapshot of the atmosphere.
Full Definition
Upper-air weather measurements made by a radiosonde, a small instrument package carried aloft by a free-flying balloon that transmits temperature, humidity, and pressure data back to a ground station as it ascends through the atmosphere. Wind speed and direction at various are derived by tracking the balloon's path. The data is used to build a vertical of the atmosphere from the up to roughly 100,000 feet.