This is a really nice explanation. I think it's helpful to go a little deeper and clarify that the virus itself isn't performing any electrical activity. Rather, the virus transfects the neural cells with a channel protein [0] that opens up in response to light. By opening the channel protein, the cell allows an influx of ions, which starts the electrical activity of the neuron in a very similar way to how it is 'naturally' stimulated. Furthermore, viral transfection is one of a few methods to expresses this protein in the animal. A population of mice that express it can be bred and maintained, with no need to transfect each one.

[0] https://web.stanford.edu/group/dlab/media/papers/Zhang%20Nat...