Plain English Meaning
How fast you are moving toward the line you are trying to join. A steep angle and high speed mean you arrive at the line quickly; a shallow angle and slower speed mean you arrive at it gradually.
Full Definition
The speed at which an aircraft is closing on a desired course, radial, or bearing during the process of joining it. It is determined by the angle of intercept and the aircraft's groundspeed, and it governs how quickly the navigation needle will center as the course is approached.
Context Anchor
Used in navigation when deciding how sharply to turn to join a course shown on an instrument, chart, or moving map.