EXAMPLE:

Imagine that you have an image that is 700px high and you do not wish it to stretch beyond 700px (otherwise it will become pixellated or blurry).
You set the (normal) Width (%) to 100% as default on MOBLE CMS.
On a Laptop the screen width is say, 900px high, therefore by setting the Max Width to 700px the image will not expand further than 700px.
However, on a mobile, that is say, 568px high, the image will render to 100% of the screen width and shrink with the size of the device.
(This is why MOBLE CMS has 'Max Width' not 'Min Width').

Given the nature of vertical scrolling 'Max Height' is infrequently used. Leave this field blank unless for a very specific purpose (e.g. thin portfolio images).