It just means that light was everywhere, and there was no shadow.
It means more than that though, it means there was no everywhere. There was no space, since as
@redfern pointed out, what does it mean to separate light from darkness, but the creation of space, with no photons in it. God is light, 1st John 1:5 (I know it's not written to you, but it's written for you, and it is profitable). How does God separate Himself (light) from ... "darkness?" What's "darkness" before Genesis 1? I don't think it exists. Darkness, as Redfern said, is space without photons. For God (Who is light) to separate light from darkness, He's going to need to create ... space. So that (for the express purpose of) there can be "darkness," whose property is space without photons, the same is also verified by both modern science and by reason.
For God to separate light from darkness, He needs to create darkness, since He is light (1Jn1:5KJV). To say that He is the otherwise inextricable combination of light and space, and that rather than darkness being a new creation, it is a metamorphosis of God, but not of God's substance, then OK, maybe that's it, but darkness is space without photons, and God is light, so before there was darkness, there was no space without light, and that doesn't actually require space at all, it could just be that God is a "light hole" as opposed to a black hole, just light, occupying no space (because there is no space required for God (literally light) to exist). He could take up no space.
Now that I think of it, darkness, and thus space, has to be created, because God being an inextricable combination of space and light, and then God metamorphosing into a spread out space (rather than all space inextricably combined with light), is more like pantheism. Like, everything's God, because God used to be an inextricable combination of space and light, but now He opened Himself up and we are all Him now, because creation was just a change in God's mode of existence.
No. God created space, and when He first removed photons from space, He created darkness. Before He separated light from darkness though, there wasn't even any logical need for space to exist, since light can just exist, taking up no space (presuming that black holes are basically this).
Could be that separating light from darkness is the creation of outer space. In such case modern science tells us it's actually still occurring, or that it at least
looks like it's still occurring. That's what the red shift indicates.