Reserva wines, is it a guarantee of anything?

I was sitting in a restaurant at the weekend waiting for friends and I heard from the next table 'Pick that one, it's a Reserva'.    It shows I suppose that the 'marketing' works, but does it mean anything?

Yes is the answer, it does mean something, but only from some countries, and it's not guaranteeing quality, it's guaranteeing the process of production, aging etc and it differs from country to country. In places like Spain, Portugal and Italy, it does mean something, BUT, and it's a big BUT, Reserva means very little in France, USA, Australia etc! It's basically a marketing tool to distinguish a producers better wine from his basic wine (in his/her opinion).

The Benchmark for 'Reserva' wines (and Crianza, Gran Reserva etc) is Spain, as that's where we see it most often here in Ireland - on Spanish wines.
It isn't, as mentioned, a guarantee of quality but of aging etc.

In Spain, Reserva is the simplest to define. All red wines from DO or DOC areas must spend a minimum of 12 months in barrel with a total required maturation period of 36 months. White Reserva wines cannot be released until they are two years old, and must have spent six months of that time in cask. Furthermore, a Reserva Cava must have spent 18 months on its lees.

This has the benefit of meaning that most are ready to drink upon release! 
You should try the Vina Alberdi Reserva 2018 Rioja, which is drinking beautifully now!

The rules vary slightly! The tightest restrictions are in the regions of Rioja, Ribera del Duero and Navarra where red Crianza wines must spend a minimum of a year in barrel with a total ageing period of 24 months.
Red wines from other areas with DO or DOCa approval need to have spent six months in cask out of the total 24 to be called Crianza.
All white Crianza wines, from any region, must have undergone a minimum of six months’ maturation in barrel, with a total ageing period of one year before release.

Gran Reserva: In Rioja, Ribera del Duero and Navarra, a Gran Reserva red wine must spent at least two years in barrel, followed by a further three years in bottle, so five years in total.

Other DO or DOCa red wines only have to spend 18 months of the five years in barrel, whilst white wines must be four years old, with at least six months in cask. Finally, a Cava has to have spent at least 30 months on the lees to be rewarded with the title Gran Reserva.

Some reputable companies from outside E.U. have adapted to the strict enforcement in the E.U. of these terms. For example, Salentein Vineyards, from Mendoza, Argentina renamed their Reserva a few years back, from Reserva to 'Barrel Selection' and the wine is as good as ever.
Try the Salentein Malbec Barrel Selection for proof!