I'm no particle physicist, but my understanding of what Krauss is saying in this book is that there were quantum fields with no particles in them - called vacuum states - which he calls 'nothing'.

His (embarrassing) interview with the Atlantic reveals this, I think.

Krauss says:

"But I am certainly claiming a lot more than just that. That it's possible to create particles from no particles is remarkable---that you can do that with impunity, without violating the conservation of energy and all that, is a remarkable thing. The fact that "nothing," namely empty space, is unstable is amazing. But I'll be the first to say that empty space as I'm describing it isn't necessarily nothing, although I will add that it was plenty good enough for Augustine and the people who wrote the Bible. For them an eternal empty void was the definition of nothing, and certainly I show that that kind of nothing ain't nothing anymore. "

And later...

Anderson: It sounds like you're arguing that 'nothing' is really a quantum vacuum, and that a quantum vacuum is unstable in such a way as to make the production of matter and space inevitable. But a quantum vacuum has properties. For one, it is subject to the equations of quantum field theory. Why should we think of it as nothing?

Krauss: That would be a legitimate argument if that were all I was arguing. By the way it's a nebulous term to say that something is a quantum vacuum in this way. That's another term that these theologians and philosophers have started using because they don't know what the hell it is, but it makes them sound like they know what they're talking about. When I talk about empty space, I am talking about a quantum vacuum, but when I'm talking about no space whatsoever, I don't see how you can call it a quantum vacuum. It's true that I'm applying the laws of quantum mechanics to it, but I'm applying it to nothing, to literally nothing. No space, no time, nothing. There may have been meta-laws that created it, but how you can call that universe that didn't exist "something" is beyond me. When you go to the level of creating space, you have to argue that if there was no space and no time, there wasn't any pre-existing quantum vacuum. That's a later stage.

Even if you accept this argument that nothing is not nothing, you have to acknowledge that nothing is being used in a philosophical sense. But I don't really give a damn about what "nothing" means to philosophers; I care about the "nothing" of reality. ***And if the "nothing" of reality is full of stuff, then I'll go with that.***