a preposition picturing one thing set around or behind another: through an opening, shut in behind a barrier, fenced about for protection, and so standing in front of someone on their behalf
This little word always pictures a barrier and someone or something on the far side of it. Its most physical use is motion or sight that passes through an opening: a spy let down through a window, a face peering through a lattice. From there it shades into the opposite idea, a door or womb shut behind a person so that nothing gets in or out.
Close that barrier all the way around someone and you get the next sense: a hedge, a wall, or a shield set about a person to keep them safe. That protective picture is what carries the word into its most common biblical use. To stand around someone as a guard becomes to stand for them, to act on their behalf. This is why the word turns up constantly with praying, pleading, and the priestly work of making things right between people and God. The one who prays for another is, in the word's own image, throwing a wall around them before God.
A last small group of uses stretches the idea of one thing standing in place of another into the language of exchange: a price paid for something, a pledge put up for a stranger. Skin for skin, as one famous line has it.
Because the same form runs from a window to a prayer, no single English word fits it. Translators reach for through, behind, about, for, and in exchange for as the verse requires.