Epoch J2000.0 is a standard point in time used as a reference in astronomy. It is precisely defined, the exact point very close to Noon, January 1, 2000 GMT.
It can be cited with the position of an astronomical object to signify an object in that position at that precise time. This allows identification of an object by position, despite movement, e.g., a planet's orbit or a star's Proper Motion. With such movement, the J2000.0 position would be extrapolated from its positions when actually observed.
Terrestrial Time (TT)