Plain English Meaning
It's a specific point in the sky that ATC uses as a checkpoint to space arriving traffic. As aircraft approach a busy airport, controllers time them so they each pass this point at the right moment, creating an orderly line of arrivals.
Full Definition
A designated point along an arrival route where Air Traffic Control measures and adjusts the flow of arriving aircraft to deliver them to a busy airport at a manageable rate. ATC assigns crossing times, speeds, or holding instructions so that aircraft cross the metering fix in a sequence that matches the airport's acceptance rate.
Context Anchor
Seen in instrument procedure and discussions, especially where points or are used to help ATC organize arriving traffic.