Imparfait = what was happening all around you (including you), background. Also ongoing events, habits, what used to be. Passé composé = what took place at that very moment: a specific event or a succession of specific events, the main storyline.
Imparfait is used for any non-specific event in the past or any repeating action in the past. I.e Je mangeais le gâteau chaque samedi. or Il allait au marché l’année dernière.
Passé composé is used for anything in the past that had a specific time. I.e J’ai mangé à midi hier.
I could go more into depth but these are the basics
