This option allows choosing the operating system which should be selected when booting. By default this always is the first one.
There are two columns because there are two ways of selecting the default operating system. The first way is to say "always select the operating system at position X". This means when a new menuentry appears, the default menuentry may change (because there's another one at Position X).
The other way is to say "use the operating system named Linux 123". Then it will always point to the same operating system - the position doesn't matter. When changing the name of an entry using Grub Customizer this option will be updated automatically. So you won't have to re-select the default entry.
