This article explains how it works:
Move from Android to iPhone, iPad, or iPod touch - Apple Support
These tips are useful, especially the bolded out one, make sure to forget all wifi connections on the Android device(so it does not reconnect without entering the password again)
If you need help with the transfer
If you have issues moving your content, there are a couple of things that you can check:
Make sure that you leave both devices alone until the transfer finishes. For example, on your Android device, the Move to iOS app should stay onscreen the whole time. If you use another app or get a phone call on your Android before the transfer finishes, your content won't transfer.
On your Android device, turn off apps or settings that might affect your Wi-Fi connection, like the Sprint Connections Optimizer or the Smart Network Switch. Then find Wi-Fi in Settings, touch and hold each known network, and forget the network. Then try the transfer again.
Restart both of your devices and try again.
On your Android device, turn off your cellular data connection. Then try the transfer again.
Good luck!