What notation is this? (Boolean/Logic Function)
You can look into this article about Boolean logic, SOP, POS, Minterms, and Maxterms:
Boolean Logic - SOP Form and POS Form

It depends whether you are looking for 1-minterm indices or 0-minterm indices.  In this case, it is using 1-Minterms since the output function is not looking for complement.   

But in general, this notation is Sigma $$\sum$$which indicates a sum of something.  Here, the something is logic terms where the output is equal to 1.  You would add all those terms together.  The Maxterms in the other hand is the opposite.  You would AND the OR terms together (POS Form).