The focal length ( f ) is the distance from a lens or mirror to the focal point ( F ). Light rays (of a single frequency) travelling parallel to the optical axis of a lens or mirror will meet at the focal point. This is shown in the figure below.

The focal points ( F ) of (left) a convex lens (with a negative focal length), (middle) a concave lens and
(right) a concave mirror. Note that a lens has a focal point on both sides of the lens axis.

Light rays that are parallel to each other, but not to the optical axis, will meet on the focal plane.