It's a song in French. It's a French song. The notes aren't. As in do re mi. It starts from do. For instance, do do dooo, rerereee, mifasoldosidoresol. And so forth.
Pont means bridge. Pont d'Avignon is, well, that. A bridge in a city called Avignon.
The story of the song goes like this. They were dancing on the bridge, "sur le pont", on the bridge.
Why? Maybe like a celebration. The legend says the rebuilt bridge was completed, using stronger material. But why does it look halfway done? Wait, it is. Yeah they say the stones were continuously collapsing each time the river was flooding, so, screw that.
Nonetheless, although it was a narrow bridge, it's still good to hold hundreds of people (before the collapse). But, jolly elephants suddenly appeared. Not really. Hence, they danced under the bridge. But then again, under that bridge is the river. I'm not sure how they danced on the river. Maybe there was something for people to stand on "beneath" the bridge.
If not, it would be quite a scene it would.
Let's sing Frère Jacques and Alouette at once.
Frère Jacques, gentille Frère Jacques
Frère Jacques, je te plumerai