WhatsApp is rolling out reservable usernames so people who do not have your number can contact you by a chosen name instead.
On most apps, a username is cosmetic.
WhatsApp built this deliberately to avoid becoming a place where strangers can find and pester you.
Here is the context the announcement leaves out: WhatsApp is not the pioneer here.
If you care about keeping your number private, do not wait for the full launch.
Read the complete breakdown, fixes and what happens next.