It basically means that one variable is linked to another in a one-way (or possibly bijective) mapping. That is, if variable $\displaystyle y$ is function of $\displaystyle x$, then choosing a value for $\displaystyle x$ will yield a (or possibly more) value(s) for $\displaystyle y$ by this particular mapping which actually is the function.

For instance, saying that $\displaystyle y$ is function of $\displaystyle x$ by the function $\displaystyle y = x^2$, we see that by choosing a value for $\displaystyle x$, we immediately obtain another value for $\displaystyle y$ which is function of $\displaystyle x$ via this function.

Does it make sense ?

I think I confused you more than helped you didn't I