Final answer:
The word 'sub' means 'under.'
Step-by-step explanation:
The word 'sub' has several meanings, but in this case, it means 'under.' When 'sub' is used as a prefix, it indicates that something is located below or beneath something else.
For example, 'submarine' means a vessel that can travel under the water, and 'subterranean' means something that is located beneath the surface of the earth.