You can set a password for any board you want to share. It's great way to keep those top secret projects private.
Set a board password
Click the Share button in the top right of your board
Enable "Read-only link"
Enable "Require a password"
Set a password. You can change this at any time
Copy the link
Edit a board password
Click the Share button in the top right of your board
Click "Change" next to the password field
Now, anyone who attempts to visit your board at the shared URL will need to enter that new password to gain access.