Within Hero Training, you can setup your heros to passively gain experience while you do other things. This works at all times even if you close the app. Please note that you can not collect experience for more than a 12 hour period. At that point the game will stop giving you experience until you collect the experience gained. There are five slots as displayed below:
You can unlock more slots at level 25, level 45 or if you buy up to VIP 5. The first three slots are classified as normal and produces 50 experience points per 10 minutes. The last two slots are VIP slots and produces 80 experience per 10 minutes.