Me teaching my SO some coding:

"Oops permission error. Run the same command but start with `sudo`."
"What does `sudo` mean?"
"Super user"
"what about the -do?"
* confused pikachu face *

As idea
sudo nano 123.txt
Equals
super user, do nano 123.txt

The su command means "substitute user" by default that is user 0, root. Starting a new shell as the substitute user.
sudo is the same but than only running/doing that one task as the substitute user instead starting a shell.