In a nutshell, multi-factor authentication (MFA) means using something else besides your password to gain access to your account. There are many ways to do this – some, such as texting a one-time PIN to your phone are less secure than others, such as using a $25 Google Titan security key or the free Authy/Twilio smartphone app. The idea is that if your password is compromised (such as a reused one that has been already leaked in another breach), your account is still secure because you have this additional secret to gain access.