Relative line-height:

Leading a.k.a line-height in web design:
The word comes from lead strips that were put between set lines. When type was set by hand in printing presses, slugs or strips of lead of appropriate thicknesses were inserted between lines of type to add vertical space, to fill available space on the page.

Source: Wikipedia
 

Absolute line-height:

Leading a.k.a line-height in web design:
The word comes from lead strips that were put between set lines. When type was set by hand in printing presses, slugs or strips of lead of appropriate thicknesses were inserted between lines of type to add vertical space, to fill available space on the page.

Source: Wikipedia