Braking is referring to what happens to the motor windings when the throttle is cut off.

If not braking, when the motor is cut, its windings are left by the ESC in a "floating" state (i.e. open circuits). This makes the rotor spin freely. With the throttle on zero, you'll feel no resistance when turning the prop by hand.

When braking, the windings are shorted to each other by the ESC, causing the rotor to halt brutally. Turning the prop by hand, you will feel a lot of resistance if braking is set.

The explanation above is true for both brushed and brushless ESC/motor combos.

Most of the time, turn off the braking. Can someone explain in which conditions is the braking useful?

Serban