The gesture feature is explained on its official manual (scroll down to Gesture), albeit slightly outdated.
What is the purpose of the small spot at the center of the screen when I turn on the app?
The small spot (or the "gesture water drop", as it's officially called) is the trigger to start gesture action.
As hinted by the error message, you have to add a new gesture for this feature to work as supposed to be:
- Open Fast Access (the drawer on the left side) by tapping the "three horizontal bars" or by sliding a finger from the left side of the screen to right.
- Open Tools, there should be a Gesture with on/off button. From here, you can disable/enable the feature. Disabling the feature will remove the water drop.
- Tap Gesture.
- Select Add a gesture.
- Choose the action you want to do.
- Draw the gesture on the middle of the dialog
- A new gesture will be added. You can also delete/edit the gesture.
- Red dot is the starting point (i.e. gesture water drop)
- Light blue line is the gesture path
Now that you have gestures defined, you can use it on the app:
- Go back to main screen
- Draw the gesture starting from "gesture water drop", following the path you've defined.