The classic plan for a first movement: present two themes, take them apart, bring them home.
Sonata form is a way of organizing a movement in three stages. The exposition presents the main ideas, usually a bold first theme and a contrasting second one. The development takes those ideas somewhere unexpected: new keys, fragments, tension. The recapitulation brings the themes back, now settled. Knowing this shape helps a listener feel where they are, the way knowing a story has a setup, complication, and resolution helps a reader.
Also seen as: sonata form, sonata-allegro form