A teaser is a short and mysterious piece of advertising created specifically to intrigue people. Imagine seeing an ad that doesn't reveal everything but only hints at something interesting. For example, you are walking down the street and see a billboard with a mysterious message: “Something big is coming…”. And that's it! No other details. That is a teaser.
The main feature of teasers is that they make us curious and keep us waiting. After such a mysterious hint, you might start googling what is going to happen or waiting for a continuation. Teasers have become a real hit in the advertising world because they help brands keep us on edge and generate additional interest in their product or service.
Example? Often, film studios release teaser trailers for their new movies. It can be just a few frames or a short clip that explains nothing but creates a special atmosphere for us to discuss and guess what will happen next.