It is convention to enter women with their maiden surname, but for more distant ancestors this is sometimes uncertain. For example, a woman might be recorded as a widow at her second marriage, but her original maiden name is unknown or based on a tentatively identified earlier marriage record.
The Surname certainty drop-down on the Naming tab of the Current Person Other Detail dialog lets you flag this uncertainty. When set, the surname is treated with caution in narrative reports and shown with a question mark on charts where applicable.