In this level we are told to use the set-uid binary to get the password to the next level.
A set-uid (or suid) binary runs as the user that owns it, even if it is executed by a different user. This binary is owned by the user bandit20 and so it will run as that user.
Executing the binary with no arguments, as we are instructed to do, gives us this:
This program runs any command we give it as the user bandit20. We can therefore use it to open /etc/bandit_pass/bandit20, which reveals the password: VxCazJaVykI6W36BkBU0mJTCM8rR95XT