At a user's detail sheet there is an option to turn geolocation on or off for the user.
If checked, when a time entry is made in the mobile FieldApp, this user will be asked the first time if the FieldApp will access their location. If yes, the coordinates of the time recording will be stored with it. This is useful for a map view because you can see where your user was physically when he or she started the time recording. You can read more about map-viewing in this help desk article.
Please note
It is possible that despite this checkbox, your user's geolocation will still not be saved when starting a time recording via the mobile FieldApp. This means that the user has denied the app access to their location. There is no way around this.