When you invest, you put time or money into something, hoping that there will be returns greater than what was originally put in.
You commonly hear talk of investing money, but you can invest time in a project as well. You can invest your hopes or emotions in a person. The money, time, or hope is called the investment. Invest can also mean to endow with a power or quality. Laws invest police officers with abilities that regular citizens do not have. Invest originally meant to clothe. In fact, there is a room in some churches still called a vestry where priests get into their garb.