Air Speed: The speed of the glider relative to the surrounding air. This naming might be misleading, it does not refer to the speed of the air, but to the speed of the glider in the air. Glider's air speed does not depend on the wind. The glider moves with the air around it.

Ground Speed: The speed of the glider relative to the ground. Wind affects ground speed: headwind slows you down, your ground speed decreases. Tailwind increases your ground speed.

Glide Ratio: The horizontal distance divided by the vertical distance the paraglider travels. You want to maximize the glide ratio if you want to fly furthest. In some cases, you want to optimize other dimensions, like speed or sync. For example, in your search for thermals, you can sacrifice your glide, and therefore the altitude, in order to cover more space in your search for the next thermal. That's a good strategy for days with strong thermals.
Once in lift, again you deviate from best glide by slowing down, so you can stay longer in lift. With no air movement, best glide can be up to about 9:1 for lower rated wings (EN A, B) and up to about 11:1 for competition gliders. For comparison, sailplanes have glides on the order of 70:1.

On the polar curve, the slope of the line from the origin to a point on the curve is the glide ratio.

Polar curve: A diagram representing the horizontal speed vs the sink rate for the glider. The pilot can adjust the position on the curve by using either brakes or acceleration system (speedbar or trims).

Stall Point: Minimum horizontal speed. From this point, if you apply any more brake, the wing will stall.

Minimum Sink: at this level of braking, the glider sinks the slowest.

Trim Speed: the speed you get when you apply no brake and no speed bar.

Top Speed: the maximum horizontal speed - speed bar is fully pressed.

Best Glide: the configuration that gives you the best glide ratio - you glide furthest. On the polar, the tangent from the origin to the curve corresponds to the best glide.