Hello! We are still taking a look at "basic" blender features, and one of them, before painting textures, animating and everything, is Retopo. When you have a model that you sculpted, usually it has a looot of polygons, at they're probably not very well distributed for being animated. That's why we create a new topology based on the shapes of the original model for creating the final animatable and manageable model :)
As you can see, retopo is actually just modeling, but making the new elements (vertex, edges or faces) snap to the base objects, so we don't have to care about the shapes, allowing us to think only on topology :) Of course, we can use this technique not only with sculpted objects, but for everything. Is extensively used for videogames; we create a High Resolution model, then we make retopo, and over the new "low poly" mesh, we project all the detail of the high poly one using maps (normal maps, AO, etc).
As always, I hope you like it!! :)