The Impressionists are the artists in France during the 1870s. It all really started with radical leanings among the painters, who came out in open opposition against academic conventionalism in art.
They wanted to get away from the classical insistence on an enclosed pictorial form, a varnished surface. They wanted their painterly technique to be open-ended, and they refused to delimit their subjects in the accepted way. This approach eventually caught on everywhere in western art, and it has left its imprint on the paintings of the 1870s and 1880s.
Auguste Renoir's La Grenouillère, the frog pond, has all these ingredients - a sketch-like painting, which to contemporaries seemed unfinished, no carved-out details, a glitter of sun reflecting the movements of the water, the boats partly truncated to convey a sense of the passing moment, and the individual details toned down in favour of the overall picture. But, the depiction of reality is still there.
Renoir has depicted an actual moment and life as it is lived, a fragment without any greater depth of interpretation. The theme is a new one: instead of something heroic, we have a casual, trivial excerpt from reality, held together by the lighting.