Plain English Meaning
If your aircraft has a collision-avoidance system and you want to fly in RVSM airspace, that system has to be a modern version that knows aircraft will be flying just 1,000 feet apart up high, instead of the older 2,000 feet. Older versions get confused by the closer spacing and aren't allowed.
Full Definition
A requirement, in airspace where Reduced Vertical Separation Minimum (RVSM) applies, that an aircraft's Traffic Alert and Collision Avoidance System (if installed) be a version capable of operating correctly with only 1,000 feet of vertical separation between aircraft above FL290. In practice this means TCAS II Version 7.0 or later, which has logic suited to RVSM separation; earlier versions can generate nuisance alerts and are not approved for RVSM operations.
Context Anchor
Seen in RVSM equipment and approval discussions, especially when checking whether an aircraft may operate in RVSM airspace.