Revenons à nos moutons literally means let’s get back to our sheep.
How could it possibly translate into English as let’s get back to the matter at hand? Should we picture a group of shepherds whiling away the morning chattering, then realizing the time and deciding they've got to get back to tending to the sheep? It turns out this idiom has a completely different origin.
To understand how sheep came to symbolize the current topic of discussion, you have to consider a piece of literature from the 15th century, La Farce de Maître Pathelin.
The eponymous protagonist of this medieval play brings two cases before a judge; one about sheep and the other about sheets (the similarity of the two words in English is entirely coincidental.) While arguing the sheep case, Maître Pathelin regularly brings up sheets in order to confuse the judge, who tries to get back to the first case each time by saying mais revenons à nos moutons. And thus, moutons came to symbolize the subject at hand.
Oui, c’est intéressant, mais ce n’est pas le but de cette réunion. Revenons à nos moutons.
Yes, that's interesting, but it’s not the purpose of this meeting. Let’s get back to the subject at hand.
Tu as encore changé de sujet – revenons à nos moutons.
You’ve changed the subject again – let’s get back on track.
Although the play was written anonymously, it’s often linked to French author François Rabelais, whose novel Gargantua and Pantagruel gave us another sheep expression: Moutons de Panurge. The idiom translates directly into English as ‘Panurge’s sheep’, with a meaning equivalent to like lemmings off a cliff, which is used to describe people who blindly follow others regardless of the consequences.
In Gargantua and Pantagruel, Panurge buys a sheep and finds out that he was overcharged for this sheep. Out of anger, he throws the sheep into the sea. The rest of the flock and the vendor himself, who has jumped on the back of the last sheep, instinctively follow suit, and all drown in the ocean.