The basic idea of quantum field theory is simple: The world is made of fields, and when we observe the wave functions of those fields, we see particles.
Unlike a particle, which exists at some certain point, a field exists everywhere in space; the electric field, the magnetic field, and the gravitational field are all familiar examples. At every point in space, every field that exists has some particular value (although that value might be zero).
According to quantum field theory, everything is a field, but when we look at a field, we see particles. When we look at the electric and magnetic fields, for example, we see photons, the particles of electromagnetism. A weakly vibrating electromagnetic field shows up as a small number of photons; a wildly vibrating electromagnetic field shows up as a large number of photons.
Quantum field theory reconciles quantum mechanics with special relativity. (This is very different from "quantum gravity", which would reconcile quantum mechanics with general relativity, the theory of gravity and spacetime curvature.)
(see also Sean Carroll: From Eternity to Here, p.269)
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