Operand

In computing, an operand is the part of a computer instruction which specifies what data is to be manipulated or operated on, while at the same time representing the data itself. A computer instruction describes an operation such as add or multiply X, while the operand (or operands, as there can be more than one) specify on which X to operate as well as the value of X.

Additionally, in assembly language, an operand is a value (an argument) on which the instruction, named by mnemonic, operates. The operand may be a processor register, a memory address, a literal constant, or a label. A simple example (in the x86 architecture) is

MOV DS, AX

where the value in register operand AX is to be moved (MOV) into register DS. Depending on the instruction, there may be zero, one, two, or more operands.
— Wikipedia

How you can describe operand purpose in computing?
Generally operand is some data on which we do some operations. Operand can be for example variable, which pointing to some data in memory.