Download All Assets
Download All Assets is an option that you can enable to download all available game assets to your device at once rather than gradually over time. This option is disabled by default, to enable it:
- Tap the Main Menu
- Tap on the Settings menu in the top right corner
- Scroll until you see Download All Assets
- Tap to start the download of the assets to your device. You can tap this option again at any time to re-download the assets to your device.
Downloading game data while connected to your Wi-Fi network will help reduce the amount of mobile data you will consume when you are exploring your local area.
Note that available game assets may change as the in-game content changes, so be sure to revisit this option regularly.
Source: Official Pokémon GO support site