Say that x is going to be equal to 1. In this case, you could say, well, x is going to vary. We can assign a value to x and see how y varies as a function of x. You can either assign variable, you can assign values to them, or you can solve for them. You could say, hey, x is going to be 1 in this case. That's not going to be the case with a random variable. A random variable can take on many, many, many, many, many different values with different probabilities.