The French phrase means "in the open air". At the end of the 19th century, painting "en plein air" was almost unheard of. The art world was dominated by salons and academies that taught traditionally approved painting methods, materials and subject matter. 

Looking for a portable way to keep his paint from drying out, a little-known American portrait painter, John G. Rand invented putting paint into little tin tubes with a screw cap. It was a defining moment. It opened the door for a group of young painters in Paris around 1868 to bring their studios into the open air, and paint entire canvases directly in nature, on-site, whether in a garden, or a café. 

For the first time in history, it was possible to unshackled artists from their grinding palettes. They were
able to experience first hand, the wind, the sun, and the mist, while trying to capture the heightened expression of nature.