CDPR says this is a technical soft-launch, but it essentially means gaining early access to The Witcher: Monster Slayer. While anyone can pre-register, it looks like only a few will be picked to be given early access to the mobile title.

From what was announced before, the events of Monster Slayer takes place long before Geralt of Rivia. So you don’t need to have played the games, read the books or watched the Netflix series to enjoy it. Understanding the lore of the universe would still help, as there will be story elements of its own. The Witcher: Monster Slayer has no release date yet, but it will ultimately be released for both Android and iOS platforms. As mentioned, this current pre-registration is only for the former, with no indication if or when there will be one for the latter. If you’re interested, you can visit the signup page linked below. (Source: CDPR via Polygon)

The Witcher  Monster Slayer Opens For Pre Registration On Android - 69