What 'unstable nothing' refers to is entities, like virtual particles, which individually have energies that are less than a quantum. For any entity to exist permanently it must at least have a quantum of energy. Sometimes, virtual particles interact and combine to have enough energy to exist, but only for a very brief time before annhilating back to the state where they aren't measurable i.e they effectively don''t exist... that is the 'nothing' part, and the fact that they pop in and out is the 'unstable' part. So, there is a quasi-state between nothing and something. That is the vacuum field or foam, depending on which theory one is using.