When a search engine tries to reach a page on your site, it is crawling. A crawler scours the internet all day every day following links and indexing them. Then, depending on how important the search engine deems your site to be and how often you update the site, the crawler decides how often it needs to visit your site.
A crawl error is when the search engine tries to reach a certain page but cannot. When this happens, the crawler cannot index the site and you will not rank.
Some sites are blocking crawlers without knowing it.