I'm reading this Wolfram page about Disjoint Unions. It shows sets $A$ and $B$ with $^*$ after them. This operator converts each element to an ordered pair and assigns $0$ or $1$ to $b$ of the ordered pair $(a,b)$.
I've not encountered this before. I Googled and found Kleene Star and free monoids but it didn't seem to match this operator, although I didn't understand the information at all!
Is there anything more to read about this $^*$ operator, or is it just some local, convenient notation on the Wolfram page?
It really is up to the writer to dictate and describe what a symbol means, because it could vary each time you encounter it.
However, in set theory, it's common use $A^*$ to denote the set of all strings that can be created using elements from $A$.
$A^*$ can notably contain the empty string, but no infinite strings.
Given the article's lack of explanation, we can only speculate as to whether or not this is what they intend. In fact, it seems like it's not the case, and I have to criticize the article for that.