Hey there,
This means that you are missing the Liberation Mono font used by the config. You could install Liberation Mono from
here, or I believe you could edit lines 581-582 to use the font of your choice.
Edit: Since you say this is your first time using emacs, I want to point out that
Casey recorded a basic tutorial about how to use emacs with his config. There is also a reference for his keybindings called "emacs_keybindings.txt" in the misc folder that you found the ".emacs" file.