An operator is a symbol that causes the compiler to take an action.
An operand that can legally be on the left side of an assignment operator is called an l-value. An operand that can be on the right side is called (you guessed it) an r-value.
Constants are r-values; they cannot be l-values. Thus, you can write
x = 35; // ok
but you can't legally write
35 = x; // error, not an l-value!