Singing and dancing means that orioles are singing and swallows are flying. It describes the noisy and active scene of birds in spring, and now it is often compared to the vigorous scene of revolution and construction.
This idiom comes from Shi's Jin Bei Ting: "The smoke is red and green, the wind is fragrant, the Yan dance is long, and the spring is long."
This idiom is generally used as a predicate and attribute in sentences.