This is a Vim syntax file for i3 config file.
You can install it using a Vim plugin manager (Vundle, Pathogen, …) or just by putting i3.vim in your ~/.vim/syntax
directory.
I recommend you to add # vim:filetype=i3
at the end of your i3 config file, unless you really want to type :set filetype=i3
every time you edit this file.