"To lock the device" means to activate a security feature that restricts access to the device, usually by requiring a password, PIN, or fingerprint to unlock it. Full definition
Other common - sense solutions include: avoiding unsecured wireless networks, locking devices with passwords and avoiding the social channel over share. (entrepreneur.com)
If you're especially worried, you can remotely lock the device and change the password. (crackberry.com)
Also, on locked devices you can't use the phone on full power and you can't change the in - built settings or default programs. (android.gs)