In America, the term is meaningless marketing and merchandising terminology that is almost always meant to convey something better than is really the case.
The term riserva or reserve is used prevalently in the wine industry. In Italy and Spain, placement of the term on the label is highly regulated and conveys a minimum amount of time the wine is aged. Because the wine is aged longer, it is often made of better quality grapes than used to make other wine. As a result, a riserva in those two countries is more expensive than the same wine lacking that designation. However, in most countries including America, use of the term is not regulated, at least as far as I can tell, so it has no more meaning than the term premium used in America.