Two-factor authentication is optional, but since Bitcoin transactions are irreversible (meaning if someone accessed your account due to a duplicate / weak password they can send your funds away), it is highly recommended to enable it on your account.
From the menu, click on SETTINGS and then on TWO-FACTOR AUTHENTICATION
Install the smartphone application
You need to either install Authy or the Google Authenticator application
Blackberry users can navigate to m.google.com/authenticator (for Google Authenticator) or sign up for an Authy account to receive authentication codes over SMS.
Once the app is installed, click the Set up two-factor auth button.
You will see a code on the screen; scan this code with your smartphone application
Print out and securely store a copy of the code for your personal records; should you ever lose your phone
Enter the code generated on your phone's application
Hit the enable button
You should be all set up with two-factor authentication!
In future when you log into your BitX account or make certain types of transactions, the system will ask for your 2FA code. Simply use your smartphone app to see what your code is (it changes every 20 seconds).
Should you ever lose or replace your phone, you can migrate your 2FA settings with Authy or do a 2FA reset.