Selection bar

If you tick this option, then the listbox will have a selection bar marking the current selected record. This is specially important when you have child boxes, as the records shown will be those related to the parent listbox, therefore the need to know which record is selected by a selection bar.

If you don't tick this option, the listbox won't have a selection bar, and you won't know which record is currently selected.

The "Color" lookup field to the right, allows you to change the color of the selection bar to be darker or lighter or whatever color you want. By default it is light blue sky.