Operand Types

• Immediate — a constant baked into the instruction.
• Register — value lives in a CPU register.
• Memory — value lives in RAM at a computed address.

mov  rax, 42            ; immediate  -> rax
mov  rax, rbx           ; register   -> rax
mov  rax, [rbx]         ; memory: load value at address rbx
mov  [rbx], rax         ; memory: store rax to address rbx

Square brackets [ ] mean "the value at this address"

— like a pointer dereference in C.