Airmail is letters and parcels sent by plane, or the system of sending them that way, as opposed to slow mail that goes by ship or truck (people still call that surface mail). As a verb, to airmail something is to send it by air. You may still see airmail stickers, thin blue envelopes, and the word on post-office forms. Though actually, these days most letters between countries already travel by air, so the old special feeling is weaker than when airmail cost extra and felt fast and a bit fancy. It is a practical postal word, a little old-fashioned in daily talk, and it has nothing to do with email. You would not airmail a message on your phone.