Re-topologising a mesh, that’s one heck of a mouthful that sounds complex and scary to people new-ish to modelling. Really it’s nothing of the kind, it’s a simple concept that’s easy to grasp and put into practice. Topology are the edge loops on your model, most people are aware that there are certain loops for example a head model should have such as loops around the eye’s, mouth, nose / chin etc. Re-topologising is simply modelling with a total disregard for edge loops or ‘good topology’ and doing what ever you need to get the shape you require then remake the mesh with GOOD topology.