Usually reserve means something is rare, special, or high quality. In this country, it is more a marketing tools than a specific or legal term.
In Italy, riserva is a legal term that only wines that are at least three years can have on their labels.
On the Iberian Peninsula, reserva is also a legal term but depending on the region the reserva wine can be 3 to 6 years old, spending much of that time in barrel but bottle age counts too.
Gran Reserva indicates wines that have been aged for six years (barrel and bottle).