Real Dictionary

build up
A build-up is a gradual increase in something.
The build-up to an event is the way that journalists, advertisers, or other people talk about it a lot in the period of time immediately before it, and try to make it seem important and exciting.
phrasal verb: If you build someone or something up, you make them seem important or exciting, for example by talking about them a lot.
phrasal verb: If you build someone up, you help them to feel stronger or more confident, especially when they have had a bad experience or have been ill.
phrasal verb: If you build up something or if it builds up, it gradually becomes bigger, for example because more is added to it.

build up in American English
to develop or increase

build up in British English
to construct gradually, systematically, and in stages
to make more desirable or attractive
noun: a gradual increase; accumulation
noun: a progressive increase in number, size, etc