The settings screen lets you do configure synchronization frequency and log out.
The frequency of automatic synchronization attemps can be set form the drop-down, where options range from one minute to one day. This affects the battery usage and bandwith consumption. The default of one hour is a sensible choice.
The log out button will log you out of the app, and request you to enter the login information again. You are not intended to log out of the application between each session. This is mostly useful when you want to log into another DHIS 2 online instance.