This is permanent. Linking binds this beta account, everything in it, and any beta cosmetics (the Beta Tester card, emblem and dig effect included) to the wallet address you pick, for good. It can't be changed or unlinked later.
Password sign-in is going away soon. Linking keeps this account's progress, unlocks the Beta Tester card, emblem and dig effect, and is what an active tester's Beta Pack at launch needs.
From then on you can sign in with the wallet as well as your password, until password sign-in goes. The wallet signs a message to prove it's yours: it never moves tokens or costs a fee.
This account and its beta cosmetics are bound to the wallet , for good, and carry into the launch. Sign in with that wallet: your password works too, until password sign-in goes away.