Crawl-Delay – What,s the meaning of this?

[lukehenry]

I hope, you all must listen about Crawl delay. Lots of webmaster use “crawl-delay” word in the robots.txt file. I searched about this, on Google and got lots answer. But, I did not get any answer, which satisfied to me. So, I made topics on this and posted here.

Sometimes, you have seen this Line in robots.txt file.
User-agent: Googlebot
Crawl-delay: 10

Can someone explain? What is the meaning above line?

[HTMLBasicTutor]

■Google does not support the crawl delay command directly, but you can lower your crawl priority inside Google Webmaster Central.

Robots.txt Tutorial - scroll down to Crawl delay

The number is time in seconds. (see above tutorial)