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 Advanced Settings
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. Downloading all assets may also improve your gameplay experience at a live event like Pokémon GO Fest or during an in-game event like Community Day.