A function consists of two values represented by an x and a y and possibly a constant.

y = 5x

y is a dependent value. Its identity, its value, all are defined by its relation to x.

x, on the other hand, is completely independent and does not need to be defined by anything but itself.

What does this have to do with anything? Nothing really, until you have to sit back and watch someone who was an x become a y.