Each Hero and almost every card has a "Target" attribute, meaning its attack or effect can only target units, structures, or Heroes that it's allowed to. This usually comes down to 3 possibilities:
Have you tried to tap on a unit only for it to seemingly not register anything? That's no bug, that means that unit can not be targeted by your Hero. Usually its for the reasons specified above (i.e. that Hero can't target Air units), but some units and structures can't be targeted until a certain action takes place.
For instance, the Shrub will only become targetable once the Pyro Troopers pop out, which happens once it gets close enough to a Hero or Structure. Another example is the Minefield, which can only be eliminated by an Airstrike or by walking over it (that's gonna hurt!).
When you're directing your Hero to move and/or attack, only one tap is required. Remember, each Hero and unit has their own speed, whether that's moving or attacking, and only one respective tap is required to get them doing what you need them to. It may seem like nothing is happening but once you tap it will either start firing on the desired target and/or move to the designated spot or target (if it's not close enough to fire upon it).
RATE OF FIRE
With that, each Hero, and unit for that matter, has its own unique rate of fire. For instance, Commando has a very fast rate of fire, while Ironhide shoots very slowly. How many times you tap on a target will not make this process go any faster. One of the keys to the game is mastering how units and Heroes attack, how far their range is, and of course, the damage they're putting out over time.
To open a chest, and claim what's inside of it, you first have to start the unlocking process. Once the timer has finished, you can then open the chest and claim the loot. Keep in mind, you can only unlock one chest a time, but can always use Gems to instantly open a chest, with no need for the unlocking process.
We hope these helped get you better acclimated to the game. Thanks for playing and have fun out there!