I can't find info in the manual or google. I want to learn about this option and what it does to shape keys? How can I use it?
One very common use is to repair a shape key from a mistake, while leaving the desired deformations intact: if you like your shape key but you see some moving vertices that should stay still, you can select them, choose blend from shape, pointing at the basis shape, unchecking the "add" option (so to replace the moving vertices informations with the basis static locations).
It looks like it's a way to copy the location of the selected vertices from another shapekey.
For example let say that it's your basis shapekey:
And this is your shapekey #1, called Key 1 by default:
If you want one vertex to go back to the location it has in the basis shapekey, select this vertex, press CtrlV > Blend from Shape:
then in the Operator box select the shapekey from which you want to copy the vertex location, tweak the Blend value:
Now your Key 1 will look like that, the vertex gets the position it has in the basis shapekey: