The A's are the aleph numbers, just using A instead of 𐡀 because I don't know how to put a subscript on the right side of an 𐡀.
They are the cardinalities of well-ordered infinite sets. Aleph-0 is the cardinality of the natural numbers, aleph-1 is the cardinality of the smallest infinite set that is larger than the natural numbers, and it goes on from there.