Steam Guard is an additional level of security introduced in 2011 by Valve to safeguard the accounts of users that make use of their digital distribution service. Once Steam Guard is enabled, access to your Steam account from an unrecognized computer requires a verification code. This means that even if hackers gain access to your username and password they will not be able to sign in to your account without the verification code that is sent to your email address. To make use of this feature you must have verified your email address and ensure that Steam Guard is active on your account.
Verify Your Email Address
Before you can make use of the Steam Guard feature, you must verify the email address that is associated with your Steam account. Because the verification code is sent to this address, it is important that you enter the correct address to ensure that you receive the mails. Sign in to the Steam client (not the Steam page in your browser) and access the settings menu to verify your email address. After you click the "Verify email address" button an email is sent to the specified address. Click the link in the email that is sent to you to verify that you are the owner of the account and have access to the address.
Enable Steam Guard
Check that Steam Guard is active and enabled by accessing the "Account" tab of your Steam settings page. A green shield emblem should display next to "Security Status" along with the words "Protected by Steam Guard since" and the date when you first activated this feature. If you don't see the shield, click the "Manage Steam Guard Account Security" button and click the radio button next to "Keep my account protected by Steam Guard."
Requesting a Verification Code
As soon as you try to log into your Steam account from a new browser or computer a code entry window opens provided your Steam Guard is active. Log in to the verified email account that is associated with your Steam account and open the email from Steam support. The subject line should be "Your Steam account: Access from new computer." In the body of the email is a five-digit verification code as well as the IP address of the computer from which you are attempting to access your account. If you do not receive the verification code within a few minutes from attempting to log in to your account, check the spam folder of your email. To get a new access code, restart your computer after exiting Steam, and then log in to your account again. Steam emails a new code to your account.
If someone manages to gain access to the login details of your Steam account, Steam Guard is the only thing preventing them from accessing your account. Ensure that the password to your email address is not the same as your Steam account as this will make it easy for intruders to circumvent the added security. Once someone has access to your Steam account they can trade away the items and games you have in your inventory or cheat in games, potentially resulting in a VAC (Valve Anti-Cheat System) ban that cannot be lifted even if you regain access of your account.