Asterisks in Jyutping Cantonese
It's not standard Jyutping, but CantoDict uses the asterisk to indicate a changed tone. In your example, waa6*2, the standard citation tone for 话 is 6, but when pronounced in the word, 广东话, its tone changes to a rising tone, so it is denoted with a *2. A note at the footer of the definition page indicates this convention:


  Also, CantoDict uses a unique "asterisk (*)" convention, to show readings such as jyu4*2. For more information please see CantoDict Tone Conventions.


Addendum: Since you mentioned sandhi, I just wanted to note that Cantonese changed tones is different from tone sandhi. This previous answer explains in further detail: https://chinese.stackexchange.com/a/9599/166