This is what your laptop displays to the left and what the video projector displays to the right.
Lets imagine you connect your laptop to a projector and you have a duplicated display.
When starting Powerpoint both the computer and the projector will show the same image.
There is a small check-box in the Slide show tab called Use Presenter View. If it is not checked then it will look like this, with the normal Powerpoint view on the local computer screen and the slides on the projector. Powerpoint will automatically switch the computer display settings to extended view when two screens are attached.
If the checkbox in the Slide show called Use Presenter View is checked then the presenter view will show up in the local computer screen instead of the normal Powerpoint edit view.
Powerpoint has now switched the computer display into extended mode instead of duplicate mode. If the presenter now starts to watch a video on the computer local screen, then that video will only show up on that screen. The audience will not see the video.
For the presenter to see what is going on the presenter either has to switch back into duplicate mode (by for example pressing Win-P and choose duplicate mode) Both the audience and the presenter can see the video.