Reddit mentions of Theoretical Nuclear Physics (Dover Books on Physics)

The usual notation for a function of more than one variable is <name>(<arg1>,<arg2>,...)

That is, B is a function of two variables, and the expression on the right is (B evaluated at Z and N) minus (B evaluated at Z and N-1).

Usually, function notation is introduced by Algebra II (or midway through the Integrated Mathematics sequence at some high schools), and the book you're reading expression appears on page 6) is for students with about the level of mathematical and physical understanding of a physics major, which includes but is not limited to multi-variable calculus, linear algebra, and ordinary differential equations.

You're way out of your depth here.

All the best stuff I learned from excited grad students who had just learned it themselves. If you can't find one of them, it is in Jackson. But oddly enough, there was this one appendix in Blatt and Weiskopf that really explained it well. IIRC, you could probably digest it after a semester of Griffiths' E&M.