In Unreal Engine, Skeleton Assets are the foundation for all animation work with Skeletal Meshes. The Skeleton Editor Mode is the visual editor where you can find the tools and properties to make changes to Skeleton Assets.
In this editor you can manipulate individual bones and bone structures, attach Skeletal Mesh Sockets, and preview any Animation Curves and Animation Notifies associated with your skeleton. The Skeleton Editor is also where you can find the Retargeting Manager, a tool for managing meshes associated with the current Skeleton Asset.