Users find them under Projects > Locked chats and open them with a four-digit account PIN; supported mobile devices can also use Face ID, Touch ID, or device unlock. The feature is rolling out gradually and is listed for Free, Go, Plus, Pro, Business, Enterprise, and Edu plans.
OpenAI explicitly says locking a chat does not create end-to-end encryption, change chat retention, or alter the “Improve the model for everyone” setting. Resetting the account PIN permanently deletes locked chats and information protected only by that PIN. The feature therefore limits casual visibility and access; it is not an encrypted vault or a retention control.