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Running off of Kyouko and from what I understand, the level curves take shape of their functions for that axis in the $xy$ plane. For example, $f(x,y)=x^2-y^2$ can be broke down as $y=x^2$ (easy visual) and $y=\sqrt x$ for $0 \leq x \leq \infty $ as $0 \geq x \geq -\infty$ can be represented with $-\sqrt {\mid x \mid}$ (also easily visualized). For the proximity of the lines from one to another, $f(x,y)$ is also recognized as $z$. The larger the magnitude of "$z$" from the set interval change should result in the lines appearing closer together on the contour plot. So like Kyouko stated, it's very similar to a topographic map with $z$ representing the elevation change.