posts from single category

Hello.

I’d like to ask, how can I have posts from single category.
You answered that in your FAQ: “For example, to limit your results to posts in category 3, you might add something like the function below to your functionality plugin:”

I’m total rookie in wordpress, but what is “functionality plugin”? is that functions.php file? Or if i have to donwload it from github (https://github.com/theukedge/functionality-plugin/blob/master/functionality-plugin.php), where should I place it?

Thanks in advance.

kindest regards