In the past, if you uploaded a video to YouTube with a video taken from your phone, you would see the black bars on either side of it with the video in the center, tall and slim. YouTube has conceded that vertical videos are here to stay and have adapted their site to accomodate them.
YouTube makes this move on the heels of Instagram introducing IGTV, a longer form of video you can create through Instagram or with their standalone app, IGTV. Instagram itself limits you to just one minute but IGTV allows you to make a video up to an hour long. Initially that will only be available to popular uploaders but will be rolled out to evveryone eventually.