$\sum$ - there just means sum. It is greek letter for S. You can rewrite the above formula as
$1 -[(e_1 - a_1)^2+(e_2-a_2)^2+(e_3-a_3)^2+\ldots ]$
$\sum$ just helps us avoid writing dozens of $+$ signs. Read more here.
What they are doing here is subtracting taking difference of expected value $e_1$ and the actual value $a_1$ for the 1st example and so on. The difference can be positive ($e_1 > a_1$) or negative ($e_1 < a_1$), so usually we square the difference to make it positive number.
The rest is there in the docs. Try putting in concrete imagined values for $a_i$ and $e_i$.