We consider a rectangular frame PQRS. Here wire QR is movable. Now we will dip the frame in the shop solution so that a soap film is formed on the frame. The film pulls the movable wire QR inward due to surface tension.
If QR is moved through a distance Z by an external force F very slowly, then some work has to be done against this force.
So, value of surface energy of liquid is numerically equal to the value of surface tension.