Grease Pencil is a particular type of Blender object that allow you to draw in the 3D space. Can be use to make traditional 2D animation, cut-out animation, motion graphics or use it as storyboard tool among other things.
The object act as a container of strokes that you can create using drawing tools in Draw Mode. More detailed editing of the strokes is done in Edit Mode, and Sculpt Mode.
To obtain best results with Grease Pencil is highly recommended to use a Graphics Tablet.