Bonjour James,
It is an action that is completed and happened once at a fixed point in time. This action is also part of a succession of actions : the person describes what happened action by action 'entendu - cru - commencé à parler'.
See link and example below:
L'imparfait sets the scene whilst Le Passé Composé relays the completed successive actions which happened once in the time frame.
Hier, il faisait beau. J'ai pris mon manteau, j'ai mis mes chaussures et je suis sorti prendre l'air.
Yesterday, the weather was nice. I took my coat, put my shoes on and I went out to get some the fresh air.
I hope this is helpful.
Bonne journée !
