Plain English Meaning
The angle between true north and magnetic north is always slowly changing. Charts and navigation systems pick a specific year as their reference and use the variation values from that year. That snapshot is the epoch year variation.
Full Definition
The magnetic variation value used on aeronautical charts and in navigation databases that is fixed to a specific reference year (the epoch year) rather than updated continuously. Because Earth's magnetic field shifts over time, the variation printed for a given location reflects the magnetic field as it existed in that reference year, and it remains in use until the chart or database is republished against a new epoch year.