Summation symbol with just one letter below it? How to verbally state. 
I'm used to seeing summation capital sigmas with a "start" at bottom and a "stop" at top and would say the sum from i to 100 or whatever. But in this case, there is just a y at bottom. 
How would I write this out so a screen reader would be able to read it aloud and get the correct idea across to the listener?

It depends on context.  Sometimes it means "sum over all possible values of $y$" (where which values of $y$ are allowed should be clear from the context), but sometimes it means an indefinite sum or "anti-difference", analogous to an indefinite integral or antiderivative:
$$ \sum_y f(x,y) = g(x,y)\ \text{where}\ g(x,y+1) - g(x,y) = f(x,y) $$
