2 Factor Authentication is an added security measure that helps ensure nobody else can log into your account if they somehow discover your login information. With 2FA enabled, each time you log in, you'll need to enter, in addition to your email address and password, a code sent to your mobile phone via sms.
When you first open your account, you'll see a notification box that allows you to enable 2FA.
After creating your account, you can turn 2FA on or off at any time when logged in by clicking on the menu icon on the top right, followed by "Settings".
Under where it says "2FA Format: +[country] [area] [number] or +44 20 71838750", enter the phone number where you would like to receive the sms message, in the format requested (country code etc.) and click "turn on" (You'll also need to enter your password). At this point a 4-digit code will be sent to your phone via sms for you to enter in. It may take a few minutes to receive the code. Once the code is confirmed, 2FA will be enabled.
You deactivate 2FA at any time by clicking "Turn off" and entering your password.