open box enclosure, open box radical (no. 17)
凵 represents an open box or container, and is primarily used as a radical component in other characters rather than as a standalone word.
凵 is a pictograph of an open container or pit. The modern form preserves the simple two-stroke outline of a receptacle.
The shape 凵 looks like an open box or a container with its top missing, ready to receive something.
For カン, imagine a can that is open at the top, just like the shape of 凵: can -> カン.