Steam Guard is a security measure that can be applied to your Steam account.
When using Steam Guard, if someone tries to log into your Steam account from an unverified computer,
they will be prompted to enter an authentication code sent to your email or from the Steam app. This helps prevent your Steam account from being easily compromised by others.
You can activate Steam Guard by logging into the Steam client,
clicking 'Steam' on the top left corner > 'Settings' > 'Security'.
For more details, please refer to the Steam website FAQ.
[Go to Steam Guard FAQ]