Current mirror circuits enable the mirroring of the current flowing somewhere else in the circuit onto a path of interest. Typically used in non intrusive load current measurements.
Current mirrorring is enabled usually with the inheritance property of BJT and FET. There are various current mirror sources from a simple opamp BJT connection to a more complicated discrete solutions with all the tolerances and fluctuations of transistors' parameters compensated for.
Current mirror circuit also finds its application in controlling amount of current into the load (Ex.: LED strips, Motors Etc) by programming a known current into one node.