What exactly is PPU and how is measured?

Hello

Please excuse me if this thread is not in the right place.

Like I say on my other thread I am new to Unity but want to create 2 D game. I make ll textures I need and now I want to start to assemble the game, but there is something I can't understand.

In every sprite, there is a PPU option. I dont know how much is one unit? Is this one square in scene view right?
When I make 315 PPU is this means 315 pixels in the square or square with sides 315x315 ?

Please help me understand. Thank you in advance.

 

This might help http://answers.unity3d.com/questions/736142/what-is-good-practice-to-set-pixels-to-units-to-an.html