If you're looking for information about the HTML5 attribute, it's called poster and the details are posted at poster.
What is a thumbnail?
A thumbnail, sometimes referred to as a thumb, is a reduced in size image that acts as a preview of a video or image. For images, it's a small, compressed version of the image. However for video, it could be a frame from the video, or it could be a custom image that has to do with the video content in some way. Thumbnails are intended to make it easier for users to visually locate video clips or images they need for their work or viewing purposes. They can also, as with YouTube, be used to attract more viewers by showing an interesting scene from the video content.
Origin of the term thumbnail
The term thumbnail came into existence for still images. It's a reference to the human thumbnail and is used to describe the small size of an image, as in it's so small it's the size of a thumbnail. This term was used in this way to refer to images back in the 17th century. However, people also applied it to anything where you were creating a smaller preview of something. It could be a painting or an essay.
Why are thumbnails important?
Thumbnails help people decide if they want to watch your content. If the thumbnail looks interesting or outrageous, people will sometimes click a video just to see what's going on and how that scene arose. Thumbnails can also give video content a completed, professional and polished look. You can choose a good image of the speaker in the video for example, or create a custom image that has to do with the content or is a title card.
Great thumbnails convey what's going to happen in the video, set the tone of the video and include your brand.
Thumbnails and api.video
api.video provides two different ways you can add thumbnails to your videos, for a professional and polished look. You can either grab the thumbnail from a frame in the timeline and automatically have the api.video API add it to the front of your video, or you can add a custom image as the thumbnail.
Learn how to create a thumbnail for a video with Python and FFMPEG here. You won't have to do this with api.video's API.
You can also see how to pick a thumbnail using the api.video API here.