What is all this information on the top left corner?

On the top left corner of the level you can see all the relevant information regarding the current checkpoint / level.

On the left side you can see the total amount of birds you have left.

Inside the blue circle you can see your current bird, and the circle itself shows you the anger meter status. Filling the anger meter will allow you to charge a bird, increasing its power.

The smaller bird shows you who is jumping into the slingshot next.

On the right side you can see the current checkpoint inside the level. Each level consists from 3-5 checkpoints, so make sure you don't run out of birds before the end!

Below the checkpoint you can see the current goals you need to fill before you move to the next checkpoint.