To enable project protection, go to ribbon – “Project” tab – “Protect Plan” and click the “Protect Editing” button. A dialog will be displayed where you can enter a password you would like to use to unlock the project in the future and specify who may unprotect the project without a password. The permissions can be set only for team members with an Editor role. Also, note that this functionality is Business only. The users with a Standard subscription can only unprotect the project.
You can also enable project protection from the Options dialog by going to File menu – Options – “Protect Plan” tab and clicking on “Protect Editing” button.
Note that if you lose or forget the password, it cannot be recovered. It is advisable to keep a list of passwords and their corresponding project in a safe place.