multivariable calculus – What does it mean to take F(x, y) as F(x, y(x))?


What does it mean to take $F(x, y)$ as $F(x, y(x))$? I just do, not, get the difference, despite numerous explanations.

To put it into context, I see this done when applying the chain rule to $F$ to find $frac{mathrm{d}F}{mathrm{d}x}$.

So what does it mean? What is the difference between $F(x, y)$ and $F(x, y(x))$?

i.e. take $F(x, y) = x^2 + y^3 + 2y$. What would $F(x, y(x))$ be equal to?

Please dumb explanations down, and don’t skip steps, as I’ve heard numerous explanations and don’t get any of them!