Scopes are a contextual hint for Sublime. You can roughly see scopes as CSS selectors. Actually, what you're seeing in the completions file is a "scope selector".
Scopes work together with syntax definitions to chop text up into named regions. Thus, Sublime can know whether the caret's in a "string.quoted.double", for example. Furthermore, it can know too that you're in a "source.python" file. So the actual scope would be "string.quoted.double source.python".
Commas in scope selectors add "or" conditions: match "text.html - source" or "punctuation.definition.tag.begin".
I might have got something wrong here, but that's the gist of it: you're refining what regions to target.
More:http://sublimetext.info/docs/extensibil ... tml#scopes