Old term used to refer to adult relations between a man and a woman. The birds reflect women, the bees reflect men, in a subtle but notable reference to sex. These days, we tend to be a lot more abrupt when discussing these issues pertaining to 'the birds and the bees'.
Why yes, son, she is a fine looking young lady. One day, I must sit you down upon on my knee and teach you about the birds and the bees.