Plain English Meaning
The speed that gets you the most height for the least distance forward. You use it when something is in the way and you need to climb steeply to get over it.
Full Definition
The airspeed that produces the greatest gain in altitude per unit of horizontal distance traveled. At this speed, the airplane climbs most steeply, clearing obstacles in the shortest ground distance after takeoff.
Context Anchor
You will see this term in takeoff performance, short-field takeoffs, obstacle clearance, and airplane manual speed charts.
Derivation
The phrase describes itself: the angle of climb is the angle between the and the ground. 'Best' here means steepest. The speed labeled VX is the one that produces that steepest angle.