It is a "preview" or "thumbnail" used to draw a visitor’s attention to a website, an image, video, or something to get a click and increase the number of clicks to the site owner.
It is very common for users to browse the internet and find really bad examples for the use of Clickbait. Hence, the general idea of this practice is pretty negative. Still, there are ways to use it without being abusive or tricky.
Clickbait is also defined as a tool used to “manipulate” or “seduce” a user, so he clicks on a site; the difference between good use of this practice and a bad one is resumed in whether you seduce the user and or trick him.
A user is deceived when the information that shows the video's progress, image, or article is unrelated or different. This could lead to the user losing interest in the article very quickly, leaving the site, losing trust in the site, and avoiding it in the future. To prevent any of these cases from occurring, the publicist needs to know that there is a lot of studies and psychology hacks behind these thumbnails, and he needs to understand how to apply them properly:
It does not matter how unethical this practice could be on this platform. It is completely valid, and users who use or abuse clickbaiting will not be punished for it.
In this platform, Clickbait's use is a little bit different from the normal websites and blogs; YouTube videos are classified not only through their name but also through the tags and the classification the creator names it. This means there are a lot of alternatives for the creators to use clickbait, this without mentioning the famous thumbnails, that are a whole other topic.
On Facebook, the case is really different; Facebook is against this practice and has been from a long time ago. They have stated that articles that “withhold information intentionally” and “mislead people, forcing them to click to find out the answer” are against their intention of creating community.
The platform has multiple mechanisms to punish and identify articles that are using this practice, some of the cases that Facebook reads as inappropriate: