If you tick this option, then the listbox for this particular procedure will print with exactly the same horizontal limits as in the window. This mean that if there are more columns than those that fit in the listbox, they won't print.
If you don't tick this option, there is no horizontal limit. This mean that if there are more columns that you cannot see in the window, they will be printed on the report. The printed listbox will horizontally grow to accommodate them. If there are other controls to the right of the listbox there will be overlapping, unless you activate the Move controls on expanded text or list option. In that case, all controls to the right of the listbox will move further to the right, and all the controls surrounding the list box will resize to fit the new size.