Signing in requires 2 elements: something you know (your password) and something you have (your phone). OTP codes are uniquely crafted for your account when you need them. They will be either sent to your mobile phone via SMS or you will get them using the Netiks OTP mobile app.
If your phone is not available, you can choose to generate backup codes and use them to login when you don't have your phone with you, for example when you are traveling. Each of these codes can only be used once. If you are running low on codes, you can always generate new codes.