Search engines such as Google look to special meta tags to help generate useful summaries of web sites to display to users in search results. For specific content (e.g. a video or an audiobook), we use the content's description as the page description for search engines. For app pages, you should write a meaningful description so users viewing the summary in search engines know what your page is about.
When writing page descriptions, keep these guidelines in mind:
Be concise: to avoid your description getting truncated in search results, try to keep it under about 150 characters.
Be accurate: describe what your page is about rather than what you think users are searching for. Inaccurate descriptions can lead to higher bounce rates when users realize the page doesn't match their expectations.
Include keywords: use words that you would search for if you were looking for a page that matched your content.
Write for humans: avoid "keyword stuffing", which is just including a long list of keywords mashed together. Write sentences a human would want to read.
Make it unique: try to write specific descriptions for each page rather than duplicating content between them.