This is a walk-with-me-while-I-do-things series, I'm going to tell you what I learned while I'm learning
tree is a command originated in MS-DOS (apparently) that has been ported to *nix systems.
By running it in the terminal, it draws for you a graphical recursive representation of the content of the current folder and its children (files and directories), and prints the number of files and/or directories listed.
It might not come with your OS, it is a program on its own. If you are on a Mac and using homebrew like me, just type
brew install tree