Frames

Add Frame (A) copies the current frame and steps forward, so the next drawing starts from what you just made. Blank Frame (N) adds an empty frame. Duplicate Frame (D) copies the current frame in place.

Play (Space) runs the scene. The bar also jumps to the first frame and the last frame, loops playback, and plays the last 10 frames (Shift+Space) or the last 50 (Ctrl+Space). Comma and period step one frame back and forward. Alt+comma and Alt+period jump to the previous and next keyframe on the active layer.

A marker flags a frame so you can find it later. Toggle Marker (Shift+M) sets or clears it, and you can pick a color from the frame menu. J and K jump to the previous and next marker.

Tweens between keys are covered in Tweening.