To keep the information on your Mac secure when you are away from it, set it to require a password when it wakes from sleep. When you return to your Mac, enter your login name and password to continue working.
On your Mac, choose Apple menu > System Settings, then click Lock Screen in the sidebar. (You may need to scroll down.)
Click the pop-up menu next to “Require password after screen saver begins or display is turned off,” then choose the time that passes before a password is required.
Locking the screen doesn’t prevent other users from turning off the Mac, restarting it, and logging in. If you think this could happen, and you use apps that don’t automatically save your changes, be sure to save your work before you leave your Mac.
You can use your administrator name and password to unlock the screen for yourself or a standard user, but not for another administrator.