F# - what does match...with | :? X as x mean?

This the Type Test Pattern.

PictureBrush is a type. shape.Fill matches :? PictureBrush when the Fill property is assignable to that type.

Also note that the as allows for binding of a variable to be of that type. So pb will be of type PictureBrush, but will have the exact same value as what shape.Fill resolved to.