Jungling is the term used to describe killing neutral minions in between the lanes and using the jungle to gank enemy heroes. Characters with Assassin, Ganker, Sieger, and Wild traits do best in the jungle, with Attacker, Burst, and Initiator are second best. A good jungler should be able to reliably kill the camps and be able to quickly kill key targets.
Jungling's main objective is killing the white camps to fill the Amber link, which gives out Card power every 2 minutes, and controlling the green buff which grants a shield to the person who last hits. Protecting your team's Amber link is important because it can be siphoned, leaving your team without Cxp. Both protecting and attacking the Amber links can lead your team to snowball faster and gain more cards and upgrades. While Jungling, controlling the river buffs from the enemy mid laner allows your midlaner to gain an advantage over the lane.
The second objective of Jungling is Ganking. Ganking is when you attack people in lane who have overextended or attacking them when least expected. While ganking, try to pick a lane with the most pressure. In example, if the safe lane is being pushed back into tower, it may be best to kill the enemy offlaner and replace the carry so they can back to base and recover for the next fight. When a team fight begins, the Jungle can be a great place to flank people retreating. Some hero's good for jungling include Khaimera, Rampage, Countess and Kwang. Some Carries or Offlaners can also jungle, like Serath or Severog.