Homebrew is what we call unofficial software made by amateur developers for closed systems such as the 3DS. This includes both games and applications, and in practice getting homebrew on your 3DS means you'll be able to :
Play out-of-region games you own.
Make your own themes to use in home menu.
Play homebrew games & apps
Preparing your SD card
In order for homebrew to run on your system, you will need to place a few files on your SD card :
boot.3dsx : this file should be placed at the root of your SD card. It's what ninjhax/ironhax/tubehax will run first ! Usually, this is the Homebrew Launcher.
3ds/ : this folder will contain all the homebrew applications you want to install to your system. You can either place 3DSX files directly inside of that folder, or create an individual folder for each application you install.
To make things easier, we put together the homebrew starter kit; you can just download it and extract it at the root of your SD card. Alternatively, you may choose to only download The Homebrew Launcher menu executable and place it at the root of your SD card.
No matter what, you should have a file named boot.3dsx placed at the root of your SD card.