Deferred (lazy) loading of YouTube videos, to improve page speed. Initially an image of the video is displayed, and the actual video is loaded and starts playing when the user clicks on it.
https://www.groupboard.com/ytdefer.html
<script src="ytdefer.min.js"></script>
<div style="width:380px;height:214px" class="ytdefer" data-alt="Enter optional img alt text here" data-title="Enter optional img title here" data-src="<youtube video id>"></div>
<script>
ytdefer_setup();
</script>
Note that by default YouTube does not always create a maxresdefault version of the thumbnail. See this answer for how to force YouTube to generate it: