Operands are the objects operated upon by the instructions.
Recall that operands may have to be fetched from a memory location to execute an operation.
Memory locations have addresses using which they can be accessed.
Operands may also be stored in the general registers.
Intermediate value of some computation which will be required immediately for subsequent computations.
Registers also have addresses.
Specifying the operands on which the instruction is to operate relates to specifying the addresses of the operands. Address can be a memory location or a register.