The Solera system (as it is called in Spain) is a finishing and blending technique to develop flavors of Sherry. By reserving a portion of a barrels older contents, you add the younger product in. This infuses the younger spirit or wine with a touch of the older more developed product, giving layer upon layer of development and character to the final product.
Imagine a pyramid of barrels stacked, with those closer to the ground being the oldest. When the most mature batches are ready for bottling, a portion of the contents are left in the cask. Then, the barrels from above are used to top off the lower barrels, with a portion of contents left inside, and the process is repeated with each layer until the top barrel is filled with new batch. This is very labor intensive, no barrel is ever emptied completely, and very little product in the Solera system will be bottled for each cycle.
That means, each year the Solera is in cycle, the output has a higher concentration of older and older product. It could contain portions of 3 year old and 20 year old Sherry in the same vessel. I imagine this takes an extraordinary amount of organization and care. Enjoying these Lustau Solera Reserva products makes me respect the process even more.
Now that you know a little about the Solera system, you will have a deeper appreciation for the Lustau Brandy and Sherry products used for my recipe: The Spanish Attic