A 2FA code is a secondary factor in authorizing a login. Depending on your chosen settings and the individual app or website, you will receive either an email or SMS text message. This is sent after you correctly enter your username and password to sign in.
You must enter this code before access is granted. In some cases, there may be a timer associated with the code. Once it runs out, the code expires. You will then need to click an option to resend the code again or repeat the entire login process.