Autoplay video is audiovisual content that starts automatically when a user goes to the host web page. Autoplay videos usually contain advertisements or promoted content. The purpose of autoplay is to get the user's attention, which it is bound to do so long as the user has sound turned on.
Like pre-roll ads, autoplay videos are a type of interrupt marketing because they come between users and the content that they have chosen to view. As such, users typically find them annoying -- which is not likely to make the ad effective. Because videos consume more bandwidth than other content, autoplay videos can also eat up a user's mobile data. Depending on the user's data plan, they could end up with charges for content that they did not request.
See a quick video how-to on disabling Facebook autoplay on an iPhone or scroll down to links with instructions for Chrome and Firefox: