Just because you make a video doesn't mean that people will watch it. With all the effort that goes into making your content it's well worthwhile to make the likelihood of people want to watch it increase, right? When a video is placed on a webpage or posted on social media the picture that you see first is called a thumbnail.
Elements of a good thumbnail
It's important that this image catches the eye, uses concise and relevant text to increase the click through rate of your video. It also doesn't hurt to use branded elements to establish the visual identity to continue building a trusting relationship with your audience. Without these elements, a lot of information is absent that could otherwise help the person determine whether there is any value for them to spend their time watching the video. Oh and not that you would, but don't show something that isn't in the video just to make people click the video.