It refers to a location or a situation that is located or happens on flat, open areas of land, typically with few trees.
Here are some examples of how it could be used:
* "Le vent soufflait fort sur les plaines." (The wind blew hard on the plains.)
* "Les chevaux galopent sur les plaines." (The horses gallop on the plains.)
* "La bataille s'est déroulée sur les plaines." (The battle took place on the plains.)
Let me know if you'd like to explore other French expressions or have any other questions.