This is an old stylized expression which is getting out of fashion. And it is used in polite written form, virtually never in conversation. The two characters mean: "descending, hanging down, from high to low" and "attention, concern."
A complete but complicated translation would be: "As an elder, superior person of higher rank, you have favored me with attention and concern." The adjectives (at least the general meaning of them) and even "you have favored me with" are omitted, but clearly understood.
Such stylized expressions are difficult for English (esp. American English) speakers. The purpose is to show respect to and to elevate the status of the person you write to.