For an instruction, the operand field specifies the data that are to be acted on by the operation. An instruction may have zero, one, or two operands
For Example :
NOP no operands : does nothing
INC AX the operand; adds 1 to the contents of AX
ADD WORD1, 2 two operands; adds 2 to the contents of memory word WORD1
In a two-operand instruction, the first operand is the destination operand. It is the register or memory location where the result is stored. The second operand is the source operand. The source is usually not modified by the instruction.
For an assembler directive, the operand field usually contains more information about the directive.