Explanation:

Box 1: var todoTable = client GetSyncTable<TodoItem>()
To setup offline access, when connecting to your mobile service, use the method GetSyncTable instead of GetTable (example):
IMobileServiceSyncTable todoTable = App.MobileService.GetSyncTable(); / Box 2: await todoTable.PullAsync("allTodoItems",todo.Table.CreateQuery()); Your app should now use IMobileServiceSyncTable (instead of IMobileServiceTable) for CRUD operations.
This will save changes to the local database and also keep a log of the changes. When the app is ready to synchronize its changes with the Mobile Service, use the methods PushAsync and PullAsync (example):
await App.MobileService.SyncContext.PushAsync();
await todoTable.PullAsync();
References:
https://azure.microsoft.com/es-es/blog/offline-sync-for-mobile-services/