It usually means either a tv show that has fewer episodes than a regular tv show or a movie that is made into parts that are essentially like episodes. Very often books are made into miniseries. For example, a show that has 4 episodes is a miniseries, but "Friends" is not a miniseries, but a sitcom (situational comedy).

A small (mini!) series on television. Longer than a film, but shorter that a TV series. Maybe about four to six episodes?

I think it is a television film that is divided into several parts. ;) Each part can be shown on a different evening.