It allows you to use a purchase made on one platform on any other platform (PC, Mac, iOS and Android). To activate it on your tablet, open the In-Game Store by selecting ‘Downloads’ and select ‘Yes’ when asked if you want to enable Cross-Purchase (if you’ve already used Cross-Sync, Cross-Purchase is enabled automatically).
This will then prompt you to sign into your Steam account on your tablet, which links that Steam account to your App Store/Google Play account. Once done, you just purchase and use items as normal and the game will automatically take care of ensuring your inventory is in-sync across all platforms.
If you encounter any issues with Cross-Purchase, feel free to post on our forums here - https://community.sigames.com/forum/653-bugs-found-on-android/