Facing the altar, the Epistle Side is the right hand side of the church. This is the side from which the lessons before the Gospel are read. In prior practice (when the Gospel was read from within the altar rail), the Gospel would then be read from the left hand side of the altar, and the lectern/bookstand holding the lessons would be moved from one side of the altar to the other. Now the Gospel is proclaimed in the midst of the people, in the middle of the nave.
In lighting church candles at the altar, candles are lit first on the Gospel side and then on the epistle side. Candles on the reredos are lit staring at the center, on the Gospel side, and then on the center on the epistle side, alternately proceeding outward. The thinking behind this is that the light of the Gospel spreads outward from the cross. Candles are extinguished in reverse order, out to in, epistle to Gospel.