An AIRPROX, short for "aircraft proximity", is an official report filed by the crew of a manned aircraft when another aircraft came dangerously close during flight. Since drones share airspace with aeroplanes and helicopters, AIRPROX reports are increasingly filed against drones too.
For a drone pilot the practical takeaway is this: if an AIRPROX has been filed against your drone, you should assume your own mandatory occurrence report is required as well. A situation the other side judged dangerous cannot simply be left unreported.