playing a video

Firstly you would need to do this using a child theme, details here:
http://codex.wordpress.org/Child_Themes

Then using the chart here:
https://developer.wordpress.org/themes/basics/template-hierarchy/
Establish that the homepage will use a specific template “home-page.php” if it is present.
Customise this page. Include your video or gif to play, AND also include the regular homepage content, use javascript to show the video/gif first and then conceal them and show the regular page thereafter.