Lesson in session
First of ,
What is smoothstep
Smoothstep is a,… well :
“Returns a smooth Hermite interpolation between 0 and 1, if x is in the range [ min , max ].”
This, according to microsoft.
Essentialy it takes your value and remaps it as if it lay between min and max instead of 0 and 1
To better visualise that, here’s a graph for smoothstep(.2,.4, (x=y)); (KINDA)
black is x=y, red is the result of the smoothstep
Sidenote: This is an artist rendition of what happens,to help visualise it when making your shaders, in reality this function is a lot more squigely and smooth, therefore smoothstep.
As you can see any value outside of .2 and .4 will become either 0 or 1.
Any value between .2 and .4 will smoothly interpolate between 0 and 1.
How to use
That is kinda up to you, but here’s how I use it:
Here, Curve will decide how far along the 0 to 1 value I animate the cutout, and
soft will decide how feathered my cutout is.
Here’s an example:
This is my x uv’s on this plane, it’s a smooth gradient from 0 to 1
This is the curve animating from 0 to 1 and back
This is soft animating from 0 to 1
And this is how powerful the combination of the two is
And here is the same thing but with a noise texture instead
Now just feed this result into your alpha channel and you are set. 
PS_ Sorry for the long post.