When your identity check passes, you are given a personal access code. It exists so you do not have to verify again: come back to the same transfer later, enter the code, and the files unlock.
Save it when you get it
It is shown to you once, at the end of verification. Put it somewhere you will find it before you close the tab.
Typing it in
The code is forgiving about formatting. It is not case-sensitive, and dashes and spaces are ignored: only the letters and digits matter, so you do not need to reproduce it exactly as displayed.
Wrong attempts are limited
Ten wrong attempts and the code locks
After ten failures the code stops working and you have to verify your identity again from scratch. Guessing is what uses up attempts, and since case and punctuation do not matter, a failure means the characters themselves are wrong, not the formatting.
If you have lost it
There is a "I lost my code: verify again" link on the entry screen. It takes you back through the identity check and issues a new code. Nothing is broken and you do not need the sender.
The code is yours alone
It is tied to you as the named recipient. Passing it to a colleague does not give them access, and sharing it is not how you forward these files: ask the sender to add them as a recipient instead.