THINK MY CONNECTION TIMED OUT AND WAS POSTED TWICE OOPS
This is my original message:
Do you mean crawl errors in Google Webmaster Tools? The most serious of these issues is the not found 404 errors - If you have a lot of these how you need to look at it is that some of your content is not accessible to Googlebot and as such you are going to affect rankings - if Google can't find your content it can't index it. Also if you have a lot of pages that are not found in Google Webmaster tools this will lead to poor user experience, frustration and a higher bounce rate for your site, which in turn will impact on your websites rankings. You need to remember Googlebot will only have a certain amount of time to crawl and index your site, if lots of this time is spent crawling and reporting errors less of your pages will be indexed and therefore you will have less pages ranking for terms in Google, once again impacting your rankings. Also if you have lots of pages that have 404 not found errors then you are potentially losing link juice if any links are pointing to these pages and they haven't been 301 redirected to tell Google of the new permanent location of the webpage.
Timed out errors also indicate that the pages listed have taken too long for Google to load, so it has given up. Once again check pages listed here and see if you can optimize them. Since Panda has come in site speed has become a factor in ranking.
Unreachable errors are those that are a result of internal server and DNS problems. Obviously these are serious as Google is having problems accessing parts of your site and once again if you don't intend to deny Google access to these pages then these errors are limiting Google from indexing these pages listed.
Good crawlability is an essential factor in user experience and search engine rankings, so take advantage of this excellent tool to make yours better if possible.