Did you see "The Girl with the Pearl Earring?" If so, perhaps you remember that she began assisting the Dutch artist, Vermeer (1632-1675), in grinding the pigments and adding the oil to make his paints.
In those days, oil painters could only sketch outdoors. The materials they needed to schlepp were too cumbersome to actually paint the scene away from their studios. Reference sketches left the artist relying on rough outlines and memory. This led to final paintings that were fanciful especially in the backgrounds and figures.
Three inventions released oil painters to paint directly from nature. They were: industrially-mixed paint in tubes, light-weight, portable easels and the speed of railroad travel.
Starting in the late 19th century,artists could gather up their supplies, board a railroad and arrive at a place in the country to paint directly on canvas - even to paint a final version.
Painting directly outdoors became known as "en plein" air or "in the open air." It was a wonderful opportunity for artists and generated some of the finest Impressionist paintings we treasure. (Monet, above; Pissarro, below left, Morisot, below right)
Today, there are plein air painting groups and workshops everywhere. So when someone tells you it is a plein air painting, you'll be able to shake your head and say, "I thought so."