After walking for a long while, the little prince comes across a road leading to a bed of roses. The little prince is devastated to discover that his rose was lying to him when she said that she was unique in the entire universe. He reflects that he is only prince over three little volcanoes and a common rose and realizes that he is not very great prince at all. He lays in the grass and cries.
The little prince's exploration has opened his brought him to the conclusion that he and his rose are not that special or unique. He thought that their uniqueness stemmed from there being nothing else like them, but now, in realizing that isn't true, he becomes sad. Much of the rest of the novel will supply a different way of thinking about what makes something unique.