I'm in construction

When a person says that they are in construction then they are saying that they work in the construction industry. It doesn’t convey any further information as to at what level in the industry, i.e. whether they own a construction business or mix muck for the bricklayer.

It is often used in social events such as parties, as a quick response to a general question, e.g. what’s your line? I’m in .......

It is also used in comedy, such as a man who works in a department store might say I’m in women’s underwear.