A kiss on the head has different meanings. It is often an affectionate, but friendly type of kiss. It is sometimes used as a starter kiss in romance, or in other cases it means thinking of someone else as just friends.
The meaning of a kiss on the head depends on how it is planted. It is sometimes used as a greeting, sometimes used to represent thinking of someone as just a friend, or it can be a starter kiss in a romantic situation. It may also represent deep affection for a loved one. A kiss on the head is affectionate, but not passionate like a romantic kiss.