The Read Range activity reads data into a DataTable object. Checking AddHeaders makes sure that columns are present in said object.
in other words
When Options - Add Headers within Read Range is selected, Column Headers are identified and are not read as a part of the data to be read from the .xls file.
When Options - Add Headers within Read Range is unselected, Column Headers are considered as a part of the data to be read from the .xls file.