You start with a few of them and the tutorial will teach you how to mindlessly spend them on nothing if don't pay enough attention (when it asks you to press "speed up" on a process that takes seconds to finish, just wait). You get them from things such as buying them with real money, sharing the game with your friends, and a few other rare occasions. The second currency of this game is gems. Of course to "encourage" "sharing" your "experience" the game will every so often slap a screen with one big button that sends a "mystery egg" to a random subset of your friends (you're given the option to dissallow the game from sending it should you be so unfortunate to hit it), but this is more or less a unsolicited spam which I refuse to send. And pretty much everything except for the most basic things require you to be in level x. You gain Xp for doing most things, such as farming, hatching eggs, and combat. You are limited to building and raising dragons based upon how much "experience" you have gained. To inhibit any decent player from being able to think for them self the game follows a very restricted linear path in the beginning. The number of farms you can have is limited to the level you're in and you may not upgrade the farms before you reach level 8.
You can then feed your dragons with the food. When the crops have grown you have to click to harvest them. To get food in the game you build a food farm. Exactly where this gold comes from is not explained (maybe they're hoarding innocent villages). A level 5 Terra Dragon produces 55 gold per minute. And then you wait for them to produce gold, out of nothing. You buy dragon eggs, wait until the hatch, and then feed the hatched dragons. Not quite sure what the definiton of an active player is, but they have about 10 million of them. Which is probably why it has 10 million "active" players.
But mostly to make you recruit everyone you know. To make you recruit everyone you know, and to make you pay $ to them. It's mechanics and functions are made for two purposes only.