This is adding a margin top which has a min value and max value. It will porportionally change between two predefined breakpoints.
This is helpful because purely using VW or VH often ends up with small screens having nearly no margin and large ones having excessive margins.
Based on Turely Fluid Typography With vh and vw Units by Michael Riethmuller .
The article mentions poor support below IE11 and Safari 8 but I've tested in IE9 and Safari 6.2 and works fine.