Free
The Free node is the default boundary condition. It means that there are no constraints and no loads acting on the boundary. When the physics interfaces is added, a default Free node is added. If you look at the selections for this node, it will show all boundaries which do not have any boundary conditions applied.
In each physics interface, the Free node applies to a geometric entity which is one level below the one on which the physics is active:
You can manually add Free nodes to override other boundary conditions. This is however seldom needed.
Location in User Interface
Context Menus
Ribbon
Physics tab with a physics interface selected: