This means, that either the feature is the only feature in the current layer or the feature is selected with the QGIS selection tool which can be found in the standard toolbar ("Selection Toolbar") or via "Edit ▶ Select".
Especially when you want to mark multiple points as significant it is useful to select multiple points. To select multiple points, press and hold the "CRTL" key (on Mac: "Command" key) and click on the features you want to select.
The plugin setting can be found under: "Settings ▶ Options... ▶ ScienceFlightPlanner".
There it is possible
to change the flight speed, the CRS used for computing sensor coverage, the setting used for computing overlap
and to add, delete or change sensors.
When the flight duration/distance display is activated (indicated by a toggled button of the corresponding feature) the distance/duration is updated in accordance to the current layer/feature selection. This means that when for example changing the layer to a different line layer, the distance/duration is updated to match the new layer. If the layer contains multiple features, it is required to explicitly select one feature for which the distance/duration is to be shown. Similarly, as for a new layer selection, a new feature selection within the current layer results in an update of the distance/duration. When a layer of a geometry other than line is selected, the display is toggled until a layer of type line is selected again.