The Free node is the default boundary condition. It means that there are no constraints and no loads acting on the edge. When the physics interface is added, a default Free node is added. If you look at the selections for this node, it will show all edges which do not have boundary conditions applied to all layers.

You can manually add Free nodes to override other boundary conditions. This is however seldom needed.

If you manually add a Free node, it will override loads and constraints on all layers from the selected edges.