OTP stands for One-Time Passcode. Your phone will generate a new 6-digit code every 30 seconds. No need to rush. You have up to 10 minutes to enter each code so you do not have to race it to press Enter before it changes.
However, once a code has been used, it cannot be used again. This means that if someone looked over your shoulder, they could not be authenticated with the code you just used.
Each subsequent code is calculated simultaneously on the server and on your phone. As a result, this app does not use any of your data plan. They will remain in sync as long as they both know what time it is.