With Steam Guard enabled, anyone attempting to login as you from an unrecognized computer or browser must first provide additional, one-time authorization. A special access code will be sent to your contact email address, and this code must be entered into Steam before your first login on an unfamiliar computer is complete. You will also be notified if any login attempts from computers other than those you've authorized occur. Steam Guard essentially acts as a form of "User Rights Management," where you as the user have greater control over access to your stuff.
To activate Steam Guard you just need to ensure you email address is verified with Steam and have downloaded the latest version. The new security feature will do the rest for you.