Focus is the point where light rays originating from a point on the object converge. The principal focus is defined as the point where a beam parallel to the principal axis appears to diverge converges from a point on the principal axis after passing through the lens.
The principal focus of Convex Mirror:
A number of rays which falls parallel to the principal axis after getting reflected, the rays seems to come from a point on the principal axis. This is called as Principal focus Convex Mirror.
The principal focus of Concave Mirror:
A number of rays which falls parallel to the principal axis on a concave mirror meets at a point on the principal axis after getting reflected. This is known as the principal focus of a concave mirror.
Check the video below for further reference.