Meaning of “build (sth) up” - Learner’s Dictionary

build (sth) up

phrasal verb with build us uk /bɪld/ verb past tense and past participle built

to increase or develop, or to make something increase or develop:

This will help to build up his confidence.
Traffic usually builds up in the late afternoon.

(Definition of “build (sth) up” from the Cambridge Learner’s Dictionary © Cambridge University Press)