Surface tension is the property of the free surface of a liquid due to which the liquid behaves as a stretched thin membrane and has a tendency to contract so as to minimize the surface area.
The tangential force per unit length on a liquid surface that acts along the normal on either side of an imaginary line on that surface is called the surface tension of the liquid.
We know that on the free surface of a liquid, the surface tension always tries to minimize the surface area. So, to increase the area of the surface of the liquid, an external force is needed. The external force doesn’t increase the area of the surface of the liquid and the work done remains stored inside the surface of the liquid as potential energy.
The surface energy of a liquid is measured by the work done to increase the area of the surface of a liquid by unity.