I can follow the explanation from the example but would still like to know what these letters stand for please which I can't find referenced in the glossary.

S
a
i

If so, Σ stands for "sum". Sum of what? Of several a_i's, where i is a generic index whose values are expected to be clear from context. Usually the first and the last values of the index are written explicitly but it is a difficult if not impossible feat in HTML.

For example if i changes from 1 to 4 then Σa_i stands for

a₀ + a₁ + a₂ + a₃.