凹 (āo) is a pictograph - the shape itself looks like a notch or dip (compare its opposite 凸 tū, 'convex/bulging'). 槽 (cáo) means 'trough'. Together: 'the dip in the trough', i.e. groove.
usage
Used in machining, woodworking, and product design. Compare with 凹陷 (a depression / indentation in a surface) - 凹陷 describes the state, 凹槽 names the feature.
