The grunts on your team (Little minotaurs) spawn at your portals. They then proceed to the enemy citadel via a specific path accross the map. That path, which intersects the path of the opponents grunts, is called a 'lane'.
You get XP in this game whenever something (Grunt, or demigod) near you dies, or you kill it. By staying near where the grunts are fighting, you get XP.
Driving someone from a 'lane' means they are no longer able to get XP for the grunts of the opponents that are attacking.
Note that experience is divided equally among possible recipients. 2 friendly DG's in a lane level at half the speed.