What is targetweight?

Animation.Blend

function Blend (animation : string, targetWeight : float = 1.0F, fadeLength : float = 0.3F) : void
Description
Blends the animation named animation towards targetWeight over the next time seconds.

Playback of other animations will not be affected.

I don't understand what this means? What exactly is this doing?

Let's say you call Blend("anim1", 0.3f, 0.5f), which will make sure that animation has weight 0.3 after 0.5. At least that's how I understand the documentation.