To kiss is to touch someone's face or lips with your own lips. Most people kiss each other as a way of saying "Hello!" or because they love each other.
You might kiss your grandma on the cheek, kiss your girlfriend on the mouth, or kiss your dog on his furry head. In each case, you're using an ancient human practice that anthropologists believe evolved either from a basic instinct, or perhaps from the habit of mothers feeding their babies pre-chewed food, mouth-to-mouth. Colloquially, you can "kiss something goodbye," or accept the fact that you'll lose it — and "the kiss of death" is something sure to bring failure.