Mirroring is a term used in two contexts. In the context of a database it normally refers to a realtime or near-realtime database replication setup. A replicated database is a configuration where changes to a masterdatabase are copied to one or more target databases.
In the context of storage, mirroring refers to a setup also known as RAID-1 where data is written to two disks at the same time. This provides redundancy and some read performance gains. The redundant copies of data mean that it can still be accessed in one of the disks fails, and the two disks allow read requests to be distributed between the disks.