What does rm -rf do?

The command rm -rf is the same as rm -r -f.

From rm's man page (type man rm in a terminal to see it) -r does:

remove directories and their contents recursively

And -f does:

ignore nonexistent files and arguments, never prompt

So in combination they do both.

In fact it is a very dangerous command because it will recursively remove everything within the directory you choose (or just remove the file you choose), and it will do so without ever prompting you.

Please use this command with care!

In addition to the previous correct answer, I would like to teach you how to fish:

When you are not sure about how a command works, what options has and what it does, open a terminal and type

man <command>

For example:

man rm

Once in there, you can search for the option. A man page can be really long to read, so in the terminal type:

/<pattern>

So for example, doing:

/-f

You can easily land to:

-f, --force
              ignore nonexistent files and arguments, never prompt

After typing /-r you'll get:

-r, -R, --recursive
              remove directories and their contents recursively

You can move between search results using n (next) and N (previous).

Bonus:

If you need to do something, but you don't know the command name, use apropos to search in man pages:

apropos <pattern>

For example:

apropos directory listing

rm is short for remove. The r flag is to remove directories and their contents recursively and the f means force, and it overrides any confirmation prompts.