What is a mesh/loop in a circuit?
Think of a mesh as 'one possible path for current to flow'.

So in a circuit with many nodes there are many possible meshes or paths. But that doesn't mean that a mesh is exclusive, no, in fact you can make up meshes or paths from other paths that exist.
In mesh analysis though, its helpful to isolate each mesh so that the current around that path is unique.

Once you have identified each mesh, apply Kirchoffs Current law and Ohms law for each loop/path/mesh and solve the simultaneous equations to find all the different currents.