Please help me understand the override system.

[angelicdezigns]

Good Afternoon, when I edit text in my define pages editor there is a warning that says

"Note: you should always edit the files located in your current template override directory, Example: /languages/english/html_includes/
Be sure to make backups after changing your files.

Editing file:
/home/sites/angelic-dezigns.co.uk/public_html/shop/includes/languages/english/
html_includes/classic/define_contact_us.php"

I have looked at the forums, and followed a link to WIKI, but I havent got a clue what any of it means.

I dont even know where to start to over ride files. I feel so silly, because everyone else obviously understands how it works, and I just cannot get my head around it at all.

Would some kind soul be able to explain it in laymans terms for this newbie please, or if anyone has found a link that I havent to an easy explanation, I would appreciate that. I feel like I am going mad

Thanks in advance.
Kind Regards
Nikki