>>13341903I don't get it but they are adding up all the (some function of I guess) for all the s in the set .
I really can't tell you what it all means beyond that I'm affraid but yeah just means the sum of all f(a)s for every a in the set A. I'm happy to have someone more knowledgeable about this specific equation, correct or explain this to me but yeah, I hope this helps :^)