I've got my radioactive source at the bottom allowing the ionising radiation to come between the plates. Each of those sparkles is representing an air particle which is being ionised. As we zoom in, the animation will show us each individual air particles and the ionising radiation coming from the bottom. It will rip an electron from the surface which is attracted towards the positive plate and the ionised air particle is then attracted towards the negative plate and drift sideways. This is happening all through the air particles and what you can see is the ionised air will drift one way and the electrons will accelerate the other way allowing the current to flow. If we zoom back out again then we will see that this causes the electric current to flow between the two which is what we've seen on the meter from the demonstration.