This diagram shows an aircraft that is in a power off descent (a glide). Since there is no power, there is no thrust and we now have only three forces acting on the aircraft.
In a steady power off descent, the combined forces of lift & drag oppose the weight and the aircraft descends at a constant speed down a straight line.
The gliding performance of an aircraft is determined by the angle between its descent path and the ground.
Since there is zero thrust and weight will not change, the glide performance of the aircraft is determined by the two remaining forces – lift and drag. The ratio of lift to drag will determine the aircraft’s glide performance – this is known as the L/D Ratio. The L/D ratio is effectively a measurement of the wing’s efficiency.
If lift is high compared to drag, the L/D ratio will be high and the glide angle will be shallow.
If lift is low compared to drag, the L/D ratio will be low and the glide angle will be steep.
As we know, the pilot can alter both lift and drag by changing angle of attack and speed. In a power off descent, the pilot controls both angle of attack and speed with the pitch attitude. Therefore, the L/D ratio will change as the pilot changes pitch attitude in a power off descent.
Your aircraft flight manual will list a best glide speed. This is the speed at which the aircraft has the maximum L/D ratio and the aircraft will glide the furthest.