You can define the summary of a page on MDN, to be used in various ways, including in search engine results, in other MDN pages such as topical landing pages, and in tooltips. It should be text that makes sense both in the context of the page, and when displayed in other contexts, without the rest of the page content.
A summary can be explicitly defined within a page. If it is not explicitly defined, then typically the first sentence or so is used, which is not always the best text for this purpose.
What is the task?
Marking the text within a page that should be used as its summary in other contexts; this task might include writing appropriate text if needed.
Where does it need to be done?
In pages that lack a summary or have a less-than-great summary.
What do you need to know to do the task?
Ability to use the MDN editor; good English writing skills; enough familiarity with the topic of the page to write a good summary.
What are the steps to do it?
Pick a page on which to set the summary:
In the MDN documentation status page, click the link under Sections for a topic that you know something about (for example, HTML).
On the topic's documentation status page, click the Pages header in the Summary table. This takes you to an index of all the pages in that topic section; it shows the page links in the left column, and the tags and summaries in the right column.
Pick a page that is missing a summary, or that has a poor summary.
Click the link to go to that page.
Click Edit to open the page in the MDN editor.
Look for a sentence or two that works as a summary out of context. If needed, edit the existing content to create or modify sentences to be a good summary.
Select the text to be used as a summary.
In the Styles widget of the editor toolbar, select SEO Summary. (In the page source, this creates a <span> element with class="seoSummary" around the selected text.)
Save your changes with a revision comment like "Set the page summary."