Tapping is a game mechanic which is used in many ways across the game. It is used to interact with objects.
Zomblin Caves, Decayed Logs, Demon Gates and Lifeless Rocks can all be attacked by single tapping them. If a dragon is available, it will come and start attacking. Multiple taps will summon several/all Dragons.
Tapping floating objects will make them drop to the nearest tile possible, if the map isn't full.
Fruit Trees, Midas Trees, Graves, Magic Coin Storage, Stone Storage, Wonders, Nests, Fountains, Cloud Mountains, Events Objects, Bushes,Cabins, Loot Orbs, Idols, and Unearthed Skeletons, all can be tapped for their products.
Coins, Bricks, Gems, Life Orbs and Bonus Points all need to be tapped in order to gather them. Higher levels of those object (with Bonus Points as an exception), however, will have a Tap Confirmation Message when are tapped (if that option is turned on in the Game Settings), and when that message (or the button on the lower left hand side) is tapped, they will be collected.
Startling Zomblins in Caves
Make it Rain
Activating Heal Extenders
Though Heal Extenders would be activated on their own after sufficient time have passed from the unlocking moment, tapping them would activate them immediately.
Harvesting an object can be performed by double tapping the object. If any dragon is available and free (not sleeping/doing something else which it autonomously chose to do) it will come and harvest the wanted object.
Quadruple (4x), Sextuple (6x) and Octuple (8x) Tapping
Some harvestable objects can have more than one dragons harvesting them in the same time. When such an object is presented, it can be quadruple tapped for calling two dragons to harvest it. If three dragons can harvest at the same time, a sextuple tap can be made to call all three. If four dragons can harvest at the same time, a octuple tap can be made to call all four.
When at least one dragon is harvesting the object, tapping the object will cause the dragon(s) to stop harvesting the object, thus double tapping cannot be used to make additional dragons to harvest the object despite still having room for more dragon(s). However, you can still drag dragons onto the object to harvest it.
In the game's Settings there's an option called "Tap Confirm". When this option is turned on, when tapping a valuable consumable object, an alert of "Confirm Tap?" would appear above the object instead of gathering it. Only after tapping the "Confirm Tap?" button, the item would be gathered. It is applies to high levels of Stone Bricks, Magic Currency, Life Orbs, Dragon Gems, Dragon Star, event points objects and various chests.
In short, this option prevent accidental unwanted taps on valuable items.