The Logger is the main way to enter your QSOs and can be opened over the Dropdown under Logbook in the Menubar at the top of the Page or over the "Add QSO" Button in the Logbook itself.
To save a QSO you need to fill at least the marked required Fields, most of thema re already filled with defaults like the ucrrent time and date. Theoretically you only need to enter the Callsign you had a contact with, select the Logbook you wan't to save it in and hit Save. If you Save a QSO it will try to automatically Upload the QSO to external Services if you enabled them by entering your credentials.
While you Type in the Callsign the logger checks if you already have the Callsign in your Log. Is that the case you will see a worked tag on the Callsign Field and informations about the last Contact on the Right side.