Original: In rondo form, a principal theme (sometimes called the "refrain") alternates with one or more contrasting themes, generally called "episodes," but also occasionally referred to as "digressions," or "couplets".
Simplified: 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".