What is a sigma rune?
I don't know exactly, but it is probably a very magical rune letter.
The topic title confuses me slightly, but in maths sigma is considered a summation sign.
Are you confusing Sigma, a Greek character, with the Sigel rune, part of the Norse Futhark? In its original language, Sigel meant the Sun, and held great importance to the Norse people. The Sigel rune was used by Nazi Germany, where it was called the Sig rune.
I'm going to go out on a limb and guess that you do indeed mean the 'Sig' rune, since as pointed out above, Sigma is a greek character and therefore, not a rune.
There are a variety of runes; the primitive alphabets coming from northern Europe, generally carved or painted on stone (no paper at that point in European history). They generally have approximately 22 phonetic characters (which means only one or two, if ANY vowels), and "sig" was the character which phonetically represented s. Its meaning in divination can vary dramatically; ranging from a lightning bolt (which was the germanic interpretation, hence its adoption by the Nazi party) to a snake to a man. Since nobody from that time period still exists, we make our best guesses, but there's no universal 100% interpretation.