Define surface energy in mathematical terms.

if its surface tension * area (surface tension = surface energy per unit area)

then y is it written in NCERT book that surface energy is surface tension per unit area.

Numerically, the surface energy is the magnitude of surface tension which is equal to the surface energy per unit area of the liquid interface. the surface energy is also defined as the force per unit length exerted by the liquid on the movable bar.

S (surface energy) = Force/ per unit length =$\frac{F}{2l}$