Programmer's Guide, Chapter 3 — The OFFSET Operator (p.60)

An address constant is a special type of immediate operand that consists of an offset or segment value. The OFFSET operator returns the offset of a memory location.

mov bx, OFFSET var    ; Load offset address

어셈블 시점에 메모리의 offset 주소를 반환한다. (바이너리에 적힌다.)