It's used to indicate a position or location that is lower than something else.
Here are some examples of words with the root "sub":
* Submarine: A vessel that travels underwater.
* Subterranean: Located beneath the surface of the earth.
* Submerge: To go under the surface of water.
* Subordinate: Someone who is lower in rank or authority.
* Substitute: To replace something with something else.
The root "sub" can also indicate a secondary or lesser degree of something. For example:
* Subspecies: A subdivision within a species.
* Subculture: A smaller cultural group within a larger culture.
* Subliminal: Below the level of conscious awareness.
Understanding the meaning of "sub" can help you understand the meaning of many words in the English language.