Dynamic Media are the different elements (texts, images, audios, videos) in a template that are variable and can be different in every final video.
Example: let's say you want to make a personalized video for every one of the contacts from a contact list exported to a Google Spreadsheet file. The file has three columns: firt name, last name, email. You want to add every person’s first name as part of the video. Some elements will be the same in every video (e.g.= your logo, a soundtrack and the background video), but other elements will change in every video, like the first name of the person. The elements that change in every video are the Dynamic Media.