MFA is a standard extra security layer which protects your account from unauthorised access. When enabled, it will send you a code on login (via text message or email) to verify it is really you.
You can choose to apply this to your own account. Simply click on your profile image (top right corner of tanooki), choose the Security option and follow the simple instructions.
If you haven’t switched it on yourself, but you’re still being asked at log-in, it means whoever has invited you to work on a Project has enforced it as part of their security protocol. You’ll be guided through setting it up and then each time you login, you’ll need to enter the code you receive via email or text message.