Compass declination is the difference between true north and magnetic north. Compasses point to magnetic north, but all good maps are based on true north. Better compasses have a declination offset adjustment- if you set this to the declination of your area, you essentially convert the compass to a true north instrument. That avoids the errors commonly made when converting from true to magnetic, and vice-versa. Declination is usually printed at the lower left on USGS maps.