In music a rondo is a piece of music which has one main theme, which is heard several times, and other musical ideas in between each time. If we give the main theme a label "A" and the other musical ideas "B", "C", "D" etc. then the form of a rondo can be described as ABACADA. The sections in between the main "A" section are called "episodes". Rondos can vary in length, e.g. a rondo might be ABACADAEAFA. Each episode is normally in a different key from the main key.
The word "rondo" is sometimes given the French spelling: rondeau.
Composers from the Baroque period onwards often write movements which are in rondo form. In the Classical music period the last movement of a symphony or sonata is very often a rondo. Rondos are normally fast and lively. They are a good way of finishing a long piece of music with something that is happy.
The word "rondo" is related to the word "ritornello" meaning: something that keeps returning.
Rondo form is sometimes combined with sonata form to make something which is called "sonata rondo form". In sonata rondo form the second tune is treated like the second subject in sonata form where it comes back in the main key the second time.