visudo is a tool for safely updating the /etc/sudoers file, found in most Linux systems (Ubuntu for example). This is the file that is required for allowing regular users run commands with superuser privileges – using sudo command.
Here's what the Ubuntu man page says about it, I think it's a great summary:
visudo edits the sudoers file in a safe fashion, analogous to vipw(8). visudo locks the sudoers file against multiple simultaneous edits, provides basic sanity checks, and checks for parse errors. If the sudoers file is currently being edited you will receive a message to try again later.
Attention: due to the sensitive content of the /etc/sudoers file, you can only run visudo as root.