Preloading the Youtube videos will use your phone/tablet device's storage space. If you don't has enough space on your device, you'd better avoid using this technique. Here is the steps on how to enable preload videos on android.
* First open the YouTube app, open the Menu and choose Settings.
* Choose "Preloading". There are two options in the page: Preload subscriptions and Preload watch later.
* Check both of them to enable, and tap OK button to proceed in the warning screen.