Añejo translates as aged or old.
For Tequila, it is a protected and regulated term. When you see it, you can be sure that the contents inside have been aged between twelve months and three years, in containers that hold no more than 600L. This is the same for Mezcal, although they are allowed to be aged in containers up to 1000L. Typically, this is done in oak barrels, although it doesn’t have to be, and producers are starting to experiment with different wood types.
Terms like Añejo are also used in rum, and largely understood to indicate an aged rum. However, there is no real clear consensus as to how long constitutes old, legal definitions for producers to abide by and so, the term has less meaning for the category.