Pretty much all websites will have crawl errors of one kind or another. These are errors with your site that Google spotted when they crawled it. Not only should you be aware of what these errors are, you should also know how to fix the most common of them.
Errors can easily creep into the thousands if your site is reasonably large. They can fall into the following areas:
Errors in sitemaps
Restricted by robots.txt
Soft 404 errors
Timed out
Not found, i.e. the page someone was trying to reach simply wasn't there
Some webmasters either aren't aware of the myriad of crawl errors that can occur, or they don't know how to fix them. Depending on the range and depth of errors that appear, it may be prudent to hire an SEO company to fix them instead of trying to figure out how to do them all on your own. It can take some considerable time to fix them all and if you have thousands you might wonder if you will ever complete them. SEO serviceshave the experience required to get the job done - very likely to a much better spec than you would do on your own.
One important area to tackle is your sitemap. Old ones should generate a 404 message for Google to see, so it can tell the difference between the old and the new. Once Google has seen the 404 message a few times it will get the message and know that this sitemap is an old one.
Obviously this is just one area you need to focus on, which is why it can be better to turn this essential work over to an SEO companyto handle for you. They can also look at handling your 404s and soft 404s. Most people are aware that a 404 page generally means the page cannot be found. The latter will often be a page that doesn't have much content on it at all. This is an easy fix and can be avoided in the future simply by fleshing out all the pages on your site.
Some people don't have the confidence to tackle their crawl errors on their own. Even if you understand how certain errors are generated - including a robot.txt timeout and a URL timeout for example - it can still be difficult to resolve them with limited knowledge. It certainly becomes more understandable that many people hire SEO services to handle these issues on their behalf. Most webmasters will benefit from focusing on other areas of their business while the experts get on with whittling down the huge lists of crawl errors for their sites.