To imprison is to hold someone in a prison or jail. It can also mean to confine them elsewhere. You might imprison a classmate in a locker, for example.
Cops imprison suspects who can't make bail, and someone could be imprisoned for a long time if convicted of a serious crime. Government agencies like the CIA and FBI also imprison people. However, you don't need a prison to imprison someone: a kidnapper holding people captive in the basement has imprisoned them.