Operand are the arguments of an instruction (ie they are the data source)
The data for a source operand can be located in:
When an instruction returns data to a destination operand, it can be returned to:
A register
An addressing mode specifies how to calculate the effective memory address of an operand.
When the value of the operand is in the instruction itself, it's called an immediate operand or immediate value.
The size of the operand
The more operands, the more complicated is the device. See wiki/Instruction_set_architecture