In most countries - USA, Australia, New Zealand, and Chile to name a few - it’s simply a marketing ploy. Producers will usually slap it on their best wines, but there’s nothing official about it. On the other hand, in Italy and Spain, the words ‘Riserva’ (Italy) and ‘Reserva’ (Spain) are legal designations. In order to include them on labels, producers need to age the wines for certain periods of time before release. That’s important because aging - especially oak-aging - has a big impact on how the wine will taste.