When you add two animations to a track of an AnimationState, eventually, the first animation will be done and the second one will start. The first and second animation can then be mixed. This allows for smooth transitions between animations. The MixDuration specifies for how long they'll be mixed.
In your code example, an empty animation is added, and the mix duration is 0. That means that the first animation will not be mixed with the second animation at all, but the transition will be abrupt.
The attachment threshold is a bit more complicated. In Spine, you can change the attachment in a slot during an animation (e.g. swap a happy mouth with an angry mouth, etc.). When you mix between two animations, each could attach a different attachment. So which attachment should win? The animation that is ending, or the animation that is starting? With the attachment threshold, you can specify when during the mixing time the attachments from the second animation should be attached.
