The Lafayette Squadron (Escadrille la Fayette) was a squadron of American pilots flying for the French during WWI. It was originally named the American Squadron. But this was before the US had entered the war, so the Germans complained.
It can be a bit hard to find. Basically, go to the Gare St. Lazare and take the “L” train to the Garches/Marnes la Coquette stop, turn left coming out of the station, and walk about 10 minutes, until you see an entrance to the park with this sign. Follow the little footpath over the river and you should see the memorial.