What these two switches decide
The mobile app works offline: it keeps a copy of the project on the phone and holds your completed forms until they are sent. Two things therefore have to happen regularly - syncing the project down (new sites, forms, tasks) and uploading your work up. By default you trigger both yourself, from the Sync sheet. Two switches under Account Settings → Sync let the app do them for you.
Which is right depends on where you work: automatic is convenient on a good connection; manual gives you control where data is expensive or the signal is patchy. This guide explains what each switch does so you - or the manager advising the team - can choose. How syncing and uploading work day to day is covered in Work offline and sync.