Plain English Meaning
It is how much lift the wing gives you for every bit of drag it costs you. A higher number means the wing is working efficiently — lots of lift, not much drag holding you back.
Full Definition
The lift/drag ratio (L/D) is the amount of lift an aircraft produces compared to the amount of drag it produces at a given angle of attack and airspeed. It is a measure of aerodynamic efficiency: a higher ratio means more lift is being generated for each unit of drag. Every airfoil has a specific angle of attack at which the L/D ratio is at its maximum (L/Dmax), and that angle is the most efficient operating point for the wing.
Context Anchor
Seen in discussions of lift, drag, gliding, , and the wing that gives the .