2FA (two factor authentication) is a vital and needed security option that everyone in today's day and age need in order to avoid being hacked and having your cryptocurrency stolen.
The most common 2FA program is Google Authenticator. You download the app onto your phone or tablet and run the app when a website asks your for your authentication code. When setting up your 2FA on a device it is recommended to have two different phones. It could be any old phone you may have lying around that you don't use anymore. That way in case you lose your phone, you have a backup. When you scan the QR code into your phone, use the other phone as well and scan it in there too. You will also be given backup codes in case you lose your phone(s). Write the backup code if you can or print it out if you have no choice and put them away in a safe place.
All online exchanges have the option for you to use it. If you join an exchange or website that does not have 2FA, RUN! On most websites you can set up your 2FA under the security settings.