Epoch simply means date and the term is used for specifying the reference date and time on which to base a Celestial Coordinate System. The current standard epoch is the j20000epoch, January 1, 2000, 11:58:55.816 UTC.
The difference between Epoch and Equinox:
Epoch gives a time when something was at a particular coordinate.
Equinox specifies the coordinates by specifying a time. Coordinates are based upon the position of visible astronomical bodies (e.g., the Sun), but these move relative to us with time in complicated ways, e.g., cycles. The equinox time specifies at what date/time we're using the positions to specify the coordinates.