Direct Line Ancestry
This tool is used to calculate the direct ancestors for a given "focus" person throughout the database. These people's names will then be flagged with the small tree icon wherever they appear in the app, as well as in the On This Day widget.
Use the Select button to select the focus person using the Lookup screen. In most cases, if this is your own family tree, you would select yourself if you want to flag all your direct ancestors. In any case, it should normally be a person near the "bottom" of the tree since it searches back in time to locate the ancestors.
When ready, tap Start calculation to begin the process. After it completes, you should see the icons appear in the various views.