The lock icon is available from multiple different places such as your profile and presentation pages as well as the deck editor. When you click on the lock you can choose whether the deck should be public or private.
Sharing a Private Deck
Even though a deck is private you can still share it with a select audience using a secret link or password protection. Learn more.
Team members are given a third visibility option: Internal. This setting can be used to make a deck visible to your team members while keeping it hidden from everyone else.