You can sign in two ways: with a password, or with a one-time code we email you. You don't have to set a password at all, since a code always works, but a password makes signing in faster.
On the sign-in page, enter your email and we send a 6-digit code to it. Type the code in and you're signed in. The code is valid for 15 minutes, and there's a 60-second wait before you can request a new one.
An email code always works, even if you've set a password. It's your way back in whenever you forget the password.
You set a password from your account page.
Once the new password is saved, every other session is signed out and this one gets a fresh token.
When you set a password, we check it against known data breaches. This is done privately, so your password never leaves your device in full. The hint under the field says "At least 8 characters. We check it against known breached passwords." If the password turns up in a breach, the field warns you as you type: "That password appeared in a known data breach. Pick a different one." And if you submit it anyway, the form answers "That password has appeared in a public data breach. Please pick a different one."
Forgot your password? You don't need to reset anything special. Just sign in with an email code, then set a new password from your account page.