A postbox is a box in a public place — on the street, outside a shop, by a station — where you put letters you want the post office to take. In Britain it is often bright red and stands like a short pillar; people also call that a pillar box. You do not open it. You push the letter through the slot, and a postal worker empties it later. This is mainly British English. In American English the usual word is mailbox, though that word can also mean the box at a house. Though actually, the post in postbox is the mail, not the post that means after. Same spelling, different word.