Plain English Meaning
The towering storm clouds that produce lightning and heavy rain. They hold a lot of electrical charge, and flying near them can cause radio noise and static on the aircraft.
Full Definition
Tall, vertically developed cumulonimbus clouds containing strong updrafts, downdrafts, heavy precipitation, lightning, and electrically charged particles. In the context of precipitation static, these clouds carry large quantities of separated electrical charge that can transfer to an aircraft flying through or near them, causing radio interference and static buildup.