What is a vacuum quantum fluctuation?

Answer 1

You could say it's a measurable fluctuation in the energy of space itself. It might be manifested in the sudden appearance of a particle or pair of particles, seemingly out of nothing, for a very brief instant.

People theorizing how this could happen usually use string theory to explain it - modelling space as an undulating surface like the top of an ocean. On our macroscopic scales, space (vacuum) appears flat and even, like looking at the Pacific from space. But on the quantum scale or smaller, space itself is supposed to be a superfine mesh of vibrating "string", the fundamental stuff making up the universe, and it has a certain amount of energy to it.

The appearance of a particle in this model would be like a droplet of water being tossed into the air from the ocean - it would appear because of the energy of the waves, from it (not from nothing), and be made of the same fundamental stuff, and quickly return to it.