The phrasal verb ‘build up’ has a number of meanings and can be used in both social and professional situations. ‘Build up' can mean the same as 'build'. You can ‘build up a relationship’ or ‘build up a reputation’, for example. When used as a noun, a ‘buildup’ can also refer to a gradual accumulation of something or increase. There might be a ‘buildup of tension’ between people who have a difficult relationship. If you are ‘building up’ to something, there is a period of excitement and preparation for something. To ‘build someone up’ is to discuss someone in a positive way in order to make others impressed, such as building up a guest before introducing them, by listing their accomplishments. You can also say you are helping to ‘build someone up’ if you are encouraging them to eat or take something to make themselves bigger, healthier or stronger - particularly after they have been sick or unwell. If you ‘build up someone's hopes’, you convince them that something will happen; when in fact it is unlikely to occur or take place.
Additional examples with 'build up' are:
An environmentalist is talking about the accumulation of harmful compounds in the environment: "CFCs have built up in the atmosphere and caused a hole in the ozone layer, which has been repairing itself since the use of CFCs has declined."
A company is discussing their impact on the local community: "We have worked hard to build up a relationship with community groups sponsoring local projects in the area, and we are creating an educational programme and work placements for university students."
A karate instructor speaks to a parent about their children competing in a tournament:
"I'd like them to compete in the competition, but I don't want to build up their hopes of winning, in case they’re not good enough."