It simply portrays some people having lunch on the grass, but for its time it was highly controversial.
It was considered something of a send-up of the bourgeoisie/ new rich of France. Think about it, it's a nice countryside setting and a picnic, but the men were in contemporary clothes, which was a major thing for the time, and the woman in front was naked and looking right back at the viewer - unlike many other female figures in art of the time, which always looked away and were kind of passive, she is a bit more assertive. Women weren't meant to be assertive at that time, and there was a distinction between 'good' women (virgins and wives) and 'bad' women (women who were sexually assertive). People thought he was glorifying bad women by painting them like this.
As an aside, that female figure in front is actually a composite - the head is that of Victorine Meurent, who posed for his other majorly controversial painting Olympia, and who herself was a painter, while the body is that of Manet's sister-in-law, and his brother is another figure in the painting.
Compared to other words of the time, this painting is also a little rough-looking and unfinished, which did not endear it to the art-buying bourgeoisie of the time.
