This little animation explores a bit of the mystery of the double slit experiment.
In one view, we think of the electrons as particles being fired one at a time from the gun, going through one of the slits, and ending up at some position on the observing screen. A flash of light is emitted at the screen when the electron hits it.
When we realise that the distribution of the flashes of light is an interference pattern, then by operational definition the electrons are waves. This view is the other part of the animation.
You may wish to muse about the fact that these two representations of the electrons are each partially correct, but neither can completely describe the experimental facts. The reality is some sort of composite of the two representations.