It's telling googlebot to slow down and not be so aggressive. You can use google webmaster tools to dial down the crawl rate. Some think you can use Crawl-del

It's telling googlebot to slow down and not be so aggressive. You can use google webmaster tools to dial down the crawl rate. Some think you can use Crawl-delay: in robots.txt but I have not tested this (and it may very well not work).