vmin & vmax are CSS thingies to tell CSS to display something depending on a length value
so a vmax value of 40 set to the height of the element will display whatever it has been assigned to as a height of 40% of the maximum length of the device viewport viewport
so, a header given a height of vmax = 40 will adjust itself accordingly (mobile phone held in portrait view will display the header as 40% of the maximum length of the device viewport, if the user changes to landscape view, obviously the maximum length viewport of the device changes so the header will automatically change to 40% of the new maximum lenth
vmin works the same way except for it calculates the height according to the MINIMUM length of the device viewport
working the two together for a background with vmin set to the height & vmax set to the MINIMUM height setting, helps stop that background image from changing too much as the viewport size increases.
take a look at the short video herehttps://youtu.be/tzo1-FUe7q4
don't concentrate too much on the other stuff, watch the background image, as it moves through the sizes, there is a little bit of movement, but not so much that a lot of the image disappears as can happen with images that haven't been given those vmin/vmax values.
The Help You Need: When You Need It The Most