The harmonium is a keyboard instrument in which the sound is produced by means of free reeds which are set in vibration by air. The accordion, for example, has a similar tone generation. By alternately stepping down two pedals, bellow is driven with the feet while playing.
The harmonium was very popular around 1900 as a home organ, as a replacement for (smaller) church organs, but also as a concert instrument. In 1945-1955 there was a temporary second renaissance as an alternative to the many organs destroyed in the Second World War.