Time - by default is connected to the comps Time attribute.
Onion Skinning - when enabled the frames before or after the current frame are visible in the viewport (ghosted).
Pre Steps - set how many previous frames to preview.
Pre Tint - set the color for the Steps.
Clear Key - create a blank keyframe or clear the contents of an existing keyframe.
Copy to Next - copy the contents of the current frame, move the timeline forward one frame and paste the contents into it.
When you create a new frame in Cel you are literally drawing a new shape every frame, not animating paths. You can copy one frame to another and then move points but each frame is a new 'drawing' and can have completely unique artwork including multiple paths if required.
If a frame is left empty it is considered a 'hold frame' and the previous frame containing any shape information will be drawn. If you want actual blank frames then you need to create one. See Clear Key above.