The drawing plane is the grey rectangle you see on the screen. On this plane, your strokes will be created in your 3D space. The way this drawing plane acts, changes depending on the camera mode you are in (please see camera modes explanation).
Move the drawing plane
The Z slider on the side of the sketch will move your draw plane through your 3D space so you can adjust where in the space your strokes are created.
Reset the drawing plane
This will move your drawing plane from its current view so that it is set to the same plane the selected stroke was drawn on. This option is located underneath the duplicate button when in Selection Mode.