"Fit" usually means scale to as large as possible within the frame whilst maintaining the original aspect ratio.
"Fill" usually means scale to the minimum necessary size to entirely fill the frame whilst maintaining the original aspect ratio.
Issue: v6 Beta (1, 2, 3) "Resize - Fill" does not (always) fill the frame (it does a standard "scale to fit") and "Resize - Fit" can scale the original to be larger than the composition (it does a standard "fill").
Note that the Layer > Video > Auto Size options are named with the usual naming convention ("Fit" fits the layer into the composition, "Fill" fills the composition with the layer).
