A tone poem is an orchestra or band arrangement that is designed to tell a story through the music. An example would be The Pines of Rome by Ottorino Respighi, in which the orchestra creates the imagery of the grand forests of the Roman Empire, or in George Gershwin's classic Rhapsody in Blue which conjures mental images of the city.
Sometimes, a piece of music is not written as a tone poem, but animators write a story that seems to match perfectly with the music. This is what Disney did in the movie Fantasia and Fantasia 2000. Although the classic The Sorcerer's Apprentice by Paul Dukas was written as a tone poem, many of the other features in the film, such as Elgar's graduation classic Pomp and Circumstance, became tone poems only after the release of the movie.
Today, most tone poems are written for high-school bands. This is because today's musicians use lyrics to tell their story. Some of the most notable tone poem composers for high-school and middle-school bands are Robert W. Smith and W. Francis MacBeth.