To kiss someone or be kissed, under a bunch of mistletoe, is a common Christmas tradition.
Mistletoe is the common name for various parasitic plants in the order Santalales.
Norse mythology tells us Frigga, Goddess of love and mother of the sun god Baldur, was alarmed by dreaming of her sons death. She drew a promise from fire, water, earth and every animal and plant not to harm her son. She overlooked however, the mistletoe. Balder was subsequently killed with a weapon made of mistletoe by Hoder, the blind God of winter.
It is said, Balder came back to life after his mother, Frigga, removed the poison of the mistltoe with her tears that turned into berries on the mistletoe. She then kissed everyone passing under the tree on which the mistletoe grew, out of gratitude and happiness.
Another thought is that the Druids considered the mistletoe to be a sacred plant never allowed to touch the ground. They believed the mistletoe had powers of fertility and healing and good health.
Some say this is how the custom of kissing under mistletoe came about. It is said, one who goes unkissed under the mistletoe will not be married the following year.