What does cache miss mean in Prerender?

Cache miss means a search bot requested a URL not yet cached in Prerender.
In this case, Prerender will render the page and serve the bot with the rendered page on the fly.
If the page has a 200 status code, Prerender will cache the page, and the next time, Prerender will serve the page from the cache.
If the page's status is not 200, for example, 404, 301, or 302, then Prerender won't cache the page after servicing the bot.

Related articles: 

- How does Prerender.io work?

- What to do if some pages are not getting cached