Meaning of “bracket” - Learner’s Dictionary

bracket

noun [ C ] us uk /ˈbrækɪt/
GROUP

a group of people whose ages, taxes, etc are between two limits:

Most heart attack victims are in the 45-65 age bracket.
SUPPORT

a piece of metal, wood, etc, that is fixed to a wall to support something, especially a shelf

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