Meaning of “under” - Learner’s Dictionary

under

adverb uk us /ˈʌndər/
BELOW THE SURFACE

below the surface of something:

The child was swimming and suddenly started to go under.
LESS THAN

less than a particular number, amount, or age:

I want a computer that is £500 or under.

(Definition of “under adverb” from the Cambridge Learner’s Dictionary © Cambridge University Press)