An accompaniment in music is music that accompanies (goes with) something else.
A piece of music may have a melody (tune) and an accompaniment underneath. The music may be played on the piano with the right hand playing the tune and the left hand playing the accompaniment. The accompaniment might also be played on a different instrument.
Music does not have to be a tune with accompaniment, although it often is. The accompaniment does not always have to be lower than the tune. To play the piano, the pianist has to learn to play a tune in the right hand and accompaniment in the left hand, or the tune might be passed from one hand to the other. The tune should usually be played a little louder than the accompaniment. The accompaniment must not ‘drown’ the tune.
An accompaniment might be single notes, or chords, or any other pattern. The accompaniment help us to feel the harmony. An accompaniment might be another tune (this is called counterpoint). Tunes can be played or sung without accompaniment. Folk songs are traditionally sung unaccompanied.
If one instrument accompanies another, the person who plays the accompaniment is an accompanist. The piano is the most popular instrument for accompanying in Western music. A good pianist can accompany a violin, cello, oboe, trumpet, singer or choir. They have to listen carefully to the instrument(s) they are accompanying, and play with the same kind of feeling.
When a soloist plays a concerto the orchestra are accompanying the soloist. An organist playing a hymn is accompanying the congregation. A percussion player in a rock band is accompanying the lead instrument.
Guitars and electric keyboards are often used for accompaniment. In Elizabethan times the lute was popular. People sang songs and often accompanied themselves on the lute or harp. In the Baroque period the accompaniment was often played by the basso continuo (harpsichord or organ with cello or bassoon on the bass line).
The pianist Gerald Moore was a famous accompanist. When he started his career in the 1920s people did not think that the accompanist was very important. Sometimes their name would not even be printed in the programme. A singer would expect the audience to start to clap as soon as they had sung their last note, even if the piano had several more bars to play. This might not matter too much with some music, but in songs by Schubert, Wolf and other composers of Lieder the piano parts are very important. Gerald Moore made people realize how important the accompanist is. A good performance can be ruined by a bad accompaniment.