Please answer, what does it mean when a spirit has crossed over into the light?

Answer 1

It's no longer bound to earth. It has moved on to heaven or whatever you want to call it.

Answer 2

That it's moved on from this world.

Usualy spirits, if remaining on Earth, have unfinished business. They're restless souls, and until that business is taken care if, their sight of the light it obscured.

When they've taken care of whatever they needed to, they can move on.