That free app is meant to make your screen emit less blue light at night. It makes your screen warmer, red/orange tint over the normal blue glow hat you'd usually see.
Some studies suggest that the blue glow emitted from computer and cellphones screens can cause your brain to delay the sleep phase, which is harmful for your health.
I did notice that when it's sunset time, I feel a bit sleepy as the screen dims to a more orangeish / flameish tone.