What is an operand?

The thing that gets operated on is called an operand.

Everything to the right of an operator is commonly referred to as the right operand of the operator.

Everything to the left of the operator is commonly referred to as the left operand of the operator.

An operator commonly operates on its right and/or its left operands to make something happen.

In Listing 3, the something that happens is to cause the right operand to be inserted into the text on the screen.