Definition - What does Solera mean?
A solera is a set of barrels which are stacked and rotated in a manner so that each of them contain similar and several multiple vintages of spirits or wine. The process of the solera system involves drawing from the oldest barrels and adding this into younger vintage barrels. That which is removed from subsequent older vintages is topped off with new or younger vintages. It is a system typical for making Sherry.
WineFrog explains Solera
The solera, or solera system is a method by which wineries who make spirits from wine or fortified wines, keep consistency from year to year.
One of the main wine regions of the world in which a solera is utilized is in Andalusia, Spain, where Jerez (Sherry) is made. However, this system is also similar in making Marsala, Madeira, Port wine and other wines and spirits. It is also used to make Balsamic and Sherry vinegars and, rum and some beer.
To simplify and understand the solera process, no barrel is ever drained. This allows some of the earlier (oldest) product to always remain. This will diminish, however there are always significant traces of the older product in the newer product being added.
Basically, the age of the product from the first vintage or bottling is the number of containers times the aging interval. As the solera matures, the average [age] approaches one plus the number of stacked rows, not including the top row. This is divided by the amount transferred or bottled.
In Andalusia, it is required by law that producers keep careful record and label the average of each container in the solera.