Question

Liquids have only one free surface. What does 'only one free surface' mean??

Answer 1

The free surface of a liquid just means the surface that is in contact with air.  Like the horizontal surface of water in a glass.  Or the top of the ocean where boats float. 

If you are on the topic of surface tension, then a soap bubble has two free surfaces, the outside and the inside.  these are free surfaces that are in contact with air. 

A bubble inside a liquid has one free surface.  That's just the place that air is touching the liquid.