To Go Out, To Turn Off
intransitive verb, ichidan verb
You've seen that 消す is "to extinguish." So, this is another verb version of that. This time, you're not doing the extinguishing, it's just happening. Something is going to disappear on its own. How can you differentiate this from 消す? Well, える allows you to notice how the object disappeared into thin air (える). You know you didn't do it, so it disappeared on its own, making it to disappear.
The reading is different from 消す, unfortunately, so you can't use that. Just think about how after the object in front of you disappears you use a key (き) to open an invisible door. That's where everything's disappearing to! You see all your disappeared stuff on the other side of this invisible door which you opened with a magical key.
WaniKani makes most of my income disappear.
My eyebrows have worn off from my sweat.
He waved goodbye until she completely disappeared from view.