Here's a primer in very basic sonata form:

Exposition: That's where you hear the main theme, which can actually be highly complex as in Beethoven's sonatas. In other words, the main theme may have sub themes.

Development: This is the section where the main theme can be modified, explored, go off into the stratosphere, become almost unrecognizable and new offshoots become developed.

Recapitulation: You're back home again. The primary theme is fully recognizable. Composers may modify the presentation as compared with the theme's appearance in the exposition, but your ear will tell you you're back home again.