We have added a new access mode for those who like vaults or those who have albums for certain people only. There may be other groups of people who will enjoy it as well. As the other access modes so also password protection is located at the bottom in the album settings with its own checkbox. After checking it, you shouldn’t forget to create a password and then save it.
When someone tries to reach your locked Photos, he shall recognize such Album by the “Password protected” label. However, any password protection is only as safe as is the password itself so choose it wisely and try to remember it.