means how many requests per second Googlebot makes to your site when it is crawling it: for example, 5 requests per second.
You cannot change how often Google crawls your site, but if you want Google to crawl new or updated content on your site, you should use Fetch as Google instead.
If Google is making too many requests per second to your site and slowing down your server, you can limit the crawl rate for root-level sites—for example, crawl rate that Googlebot should make.
Note that it does not guarantee that Googlebot will reach this maximum rate.
We recommend against limiting the crawl rate unless you are seeing server load problems that are definitely caused by Googlebot hitting your server too hard.
You canot change the crawl rate for sites that are not at the root level—for example, The new crawl rate will be valid for 90 days.
If your crawl rate is described as "calculated as optimal" on the site settings page, the only way to reduce the crawl rate is by filing a special request.
However, the thumbnail in google webmaster doesn't change.
It still shows my old website layout and in google search, it still shows my old website description.
More info: It was only about 1 day ago since I made the changes.
Question is, will it take time for the google webmaster to update my account as its only 1 day since I made the change? I'm not sure if I have a source for this, but I think it can take a few days or more to update.
I had a similar situation where I made a site live and submitted the sitemap, then tried changing things over the weekend to improve seo and I saw nothing until about 3 days later.
I wondered if I was violating something since I saw no changes right away.