Precise url path for files

[emailapphost]

I am working to move my large video files (100+ mb files) from the db to the files system, in order to make it hopefully work.

In the Attachments manager is says:
Attachment File PathThis file path should be readable AND writeable by your web server (usually chmod 0777)

Then it states the file path as:
/path/to/your/attachment/directory

What does this mean, precisely? (excuse my ignorance).
What is the url for this?
Do I need to write out the entire URL here, or just the local path?

http://www.mydomain.com/directory
http://www.mydomain.com/home/
http://www.mydomain.com/public_html/forum/directory