Define the principal focus of a concave mirror.

Light rays that are parallel to the principal axis of a concave mirror converge at a specific point on its principal axis after reflecting from the mirror. This point is known as the principal focus of the concave mirror.

A parallel beam of light after reflection from a concave mirror converges at a point in front of the mirror. This point (F) is the focus of a concave mirror and it is real. 

The number of rays parallel to the principal axis are falling on a concave mirror which meat at a point is called principal focus of the concave mirror.