Lvalue or location value is an address that is stored into while rvalue is the "read" value, ie., value of a constant or the value stored at a location. In the assignment
a = 31;
we have the lvalue of a used on the left hand side and the integer contant value 31 assigned to this location.
In the assignment
b = a;
we have the rvalue of a (31) used to assign to the lvalue of b . So depending on which side of an assignment, we use the variable a , we get two different behaviors.