Digital voltmeters, frequency counters etc use 7 segment displays to show the
result of measurements.
However all the internal electronics use binary for
manipulation of the data.
To convert this binary to a form that can be used by the display
requires the use of a DECODER, in this case a "binary to 7 segment display
This has four inputs for the binary and seven outputs for the display.
Other types of decoder are available such as binary to decimal.