The birds and the bees

"The talk," also known as "The birds and the bees", is an American colloquial expression referring to a rite of passage in the lives of most children when parents explain human sexuality and sexual intercourse to them.

Meaning
According to tradition, "the birds and the bees" is a metaphorical story sometimes told to children in an attempt to explain the mechanics and results of sexual intercourse through reference to easily observed natural events. For instance, bees carry and deposit pollen into flowers, a visible and easy-to-explain parallel to fertilization. Female birds laying eggs is a similarly visible and easy-to-explain parallel to ovulation.

Origin
While the earliest documented use of the expression remains somewhat nebulous, it is generally regarded as having been coined by Samuel Taylor Coleridge. Published in 1825, his verse in “Work Without Hope”, refers to birds and bees.

All Nature seems at work. Slugs leave their lair— The bees are stirring—birds are on the wing— And Winter, slumbering in the open air, Wears on his smiling face a dream of Spring!