The bright lines on the screen are where the tops of the waves joined each other (constructive interference) and then made it to the screen. The dark spaces in between are where the top of one wave hit the bottom of another wave (destructive interference), canceling them both out and never making it to the screen.

So, when we send “particles of matter,” like the BB’s, through two slits, we get two definite patterns on the screen that look like the slits they came through. When we send “waves” through two slits, we get an interference pattern on the screen.

Simple enough. Now let’s try this experiment with electrons instead of BB’s….