1der1 posters are pictures in front of embedded iframes (an iframe is a window within a web page, which contains another web page). This allows you to use an attractive, informative still image. Pictures are loading faster than iframes. Posters are typically used for video iframes, but can be used for any iframe. On the other hand, you can use videos with posters but without iframes.
If you use posters for iframe embedded videos, you are typically choosing thumbnails of the video.
Just paste your Vimeo, Wistia or YouTube video embed code and click the button to show existing thumbnails.