You can read more about Kefir ID in this article

By linking together your progress and Kefir ID you’ll get a small gift. This way of connection will only work with a Google account (mail ending in / with Apple-ID. Please note that if your progress is already connected, then you do not need to disconnect and link anything again! You will simply receive the item (in the "Inbox" section) immediately upon entering the game after installing a fresh update. Make sure you have the latest version of the game if this does not happen. If you are unable to connect the progress itself, then the following steps may help:

- clear the cache of the game and the Internet browser on your device;
- change the Internet browser that is used on your device by default.

If (when you click on the "Connect" button with an exclamation mark icon and enter account data) the game invites you to download another progress, it means that you have previously connected another progress to this account. In this case, you just need to link the game to another mail. You can receive such a gift for your progress only once.