If someone or something is dear to you, it means you hold them or it very close to your heart, as in "My country is very dear to me" or "She is a dear friend."
As a written form of address — such as "Dear Mr. So-and-so" — dear is generally a polite but impersonal standard greeting. Dear can sometimes mean expensive, as in "The cost of food is so dear these days," though that's a rather dated usage nowadays. Used about children, baby animals, or other examples of insufferable cuteness, dear can also mean "sweet" or "adorable." In this sense, dear is a favorite word of grandmothers and other doting relations.