A skeleton is a nearly finished solution in a Fewest Moves solve. The intent behind a skeleton is to do the majority of the solve, then insert the final moves somewhere within the skeleton that give fewer moves than performing a different algorithm at the end. A common technique is to create a skeleton leaving 3-5 corners, then review their position throughout the solution to find the optimal place to insert (an) 8-move commutator(s) that, ideally, cancel moves within the skeleton.
While 3-5 corners is the most common skeleton configuration, a skeleton could essentially take any form--such as leaving a three edge cycle. However, corner cycles are generally considered the most effective, largely because Fewest Move solves are judged in HTM, and edge cycles depend on slice moves.