Teasers are sometimes displayed associated with links to the page, they are not displayed on the page.
A teaser may be the same as the summary. Separating the teaser and summary increases flexibility.
A teaser is unique and sufficient to understand the content alone. For example 'find out if you are eligible and apply for a grant' is not sufficient. It needs more detail and the following is better 'find out if you are eligible and apply for a grant to plant trees on farmland'.
Along with the title, teasers help users quickly understand whether the page has the information they want.
Aim for a maximum of 160 characters.
Teasers end with a full stop.
Teasers are information not already provided by the title.