Classical hand-drawn animators often made a chart, showing exactly when each drawing, sound and camera move would occur, and for how long. They nicknamed this the 'dopesheet'. While CG foundations dramatically differ from classical hand-drawn animation, Blender's Dopesheet inherits a similar directive. It gives the animator a 'birds-eye-view' of every thing occurring within a scene.
There are four basic views for the Dopesheet. Action Editor, Dopesheet, Shape Key, and Grease Pencil. These all view different contexts of animation.