no, it has nothing to do with tax...
'retail' is the price the store sets for the book [not including tax, which varies from place to place], as opposed to what they paid the publisher for it [= 'wholesale']...
'net' is the total amount the publisher makes from selling your book, minus the cost to them, of publishing it...
but, it's still not that simple... see this very good rundown on royalties, for the devil in the details:
http://www.ivanhoffman.com/royalties.html