Plain English Meaning
Some airports are tricky enough — because of mountains, obstacles, or unusual procedures — that the FAA requires the captain to have specific knowledge of that airport before flying there. The captain proves this either by having flown in there recently or by completing an approved briefing or training package on that airport.
Full Definition
A regulatory requirement under 14 CFR 121.445 that obligates pilots in command operating into certain designated airports — those with surrounding terrain, obstructions, unusual approach or departure procedures, or other special characteristics — to demonstrate familiarity with that airport before flying there as PIC. Qualification is met either by making an entry into the airport (in either pilot seat or as an observer) within the preceding , or by completing an approved pictorial means covering the airport's special features. The FAA publishes the list of designated special airports.