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.You can manually add Free nodes to override other boundary conditions. This is however seldom needed.Physics Tab with a Solid Mechanics interface selected:
Boundaries>Solid Mechanics>Free