isRowSelected() is used in a multi-row selection mode to check whether a row is selected. The method is typically used in a conditional statement and needs two arguments:
IF dlg.IsRowSelected ("screen_array", index) THEN...
isRowSelected() returns a Boolean value, which is TRUE if the specified row is selected, and FALSE - if not.
index specifies the index of the row to be checked.
There is some specifics about using variables with ui.Dialog methods.
You can learn about them here.