I read in the book that etwas means some.
- Ich brauche etwas frisches Fleisch.
- Er hat etwas Geld.
There is also another determiner that is einige. It also means some.
- Vor einiger Zeit schon.
- Mit einigem Glück.
Are they interchangeable?
I'm a native German speaker and I noticed that this is quite a hard question, but let my try to figure out a "rule" here (although I don't think there is a real "static" rule):
Sometimes you can use both "einige" and "etwas", i.e. you can say "Mit einigem Glück" (which means you need quite some luck) and "Mit etwas Glück" (-> a little luck...) but in those cases, "etwas" is less than "einige".
But the main thing is that, for abstract quantities (i.e. things you cannot count) you use "etwas" and for specific quantities you use "einige". For Example you would say
(since you cannot "count" meat) but you would say
(because you can count pieces of meat)...
As I said there certainly will be exceptions to those rules!
EDIT: I just thought about the following: It's a little bit like "which" and "what" in English.
This seems to me to be much like the distinction between "less" and "fewer" in English: http://www.oxforddictionaries.com/words/less-or-fewer