eft is your number of remaining birds to shoot, which puts a limit on how many attempts you can make at toppling your target.
As you beat levels the next ones unlock, for a total of 15 in which the number of bad guys and platforms increases every time.
Angry Parrots - Slingshot Game! closely follows the idea of Angry Birds, both in terms of the gameplay and character design, but without the bonuses, different types of birds to shoot, and other options that complicate your playing.