several characters have focus spells. They mean you initially cast the spell, and as long as you don't take any other actions (move, cast a spell, etc) or have anyone interrupt you, you "recast" the spell automatically every few seconds and your power goes down each pulse.
In the case of the heretic, the damage increases with each pulse, so for instance, you might hit for 100 damage, 4 seconds later, hit for 200 damage, 4 seconds later, hit for 300 damage, 4 seconds later, hit for 400 damage. This is assuming you had enough power to last these 4 pulses, and that you didn't take any other actions or get interrupted.