Maybe a mathematical example will make things clearer. Let's say we have the function:\\
$f(x, y) = y / x$
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In order to evaluate the function for $x = 2$ and $y = 3$ we would do:\\
$f(2, 3) = 2 / 3$\\
and be done.
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However, how about we just put in x first and make a new function. Since x is gone, we can write:\\
$g(y) = f(2, y) = y / 2$
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And in a second step we solve the function $g(y)$:\\
$g(3) = f (2, 3) = 3 / 2$