Posters are pictures in front of embedded iframes (an iframe is a window within a web page containing another web page). A thumbnail allows you to use an attractive, informative image. Pictures load faster than iframes. Typically, you use posters for video iframes, but you can use them for any iframe. On the other hand, you can use videos with a thumbnail but without an iframe.
If you use posters for iframe embedded videos, you normally choose thumbnails of the video.
Paste your Vimeo, Wistia, or YouTube video embed code and click the button to show existing thumbnails.