Plain English Meaning
It is the fastest speed a pilot is allowed to fly while circling in a holding pattern. The limit changes depending on how high the aircraft is flying.
Full Definition
The highest indicated airspeed at which an aircraft may be flown while in a published or assigned holding pattern, expressed by the FAA in altitude bands: 200 KIAS at or below 6,000 feet MSL, 230 KIAS from above 6,000 feet up to and including 14,000 feet MSL, and 265 KIAS above 14,000 feet MSL, with specific exceptions for certain holding patterns and aircraft types.
Context Anchor
Seen in instrument flying when entering, flying, or planning a holding pattern on an approach chart or under an .