When first learning how to use Grid Layout, you might begin by addressing positions on the grid by their line number. This requires that you keep track of where various lines are on the grid, and also be aware of the fact the line numbers reverse if your site is displayed for a right-to-left language.
Built on top of this system of lines, however, are methods that enable the naming of lines and even grid areas. Using these methods enables easier placement of items by name rather than number, but also brings additional possibilities when creating systems for layout. In this article, I’ll take an in-depth look at the various ways to name lines and areas in CSS Grid Layout, and some of the interesting possibilities this creates.
Source: Smashing Magazine Naming Things In CSS Grid Layout