Web crawlers, like spiders or bots, are software programs that search engines use to scan and analyze websites. They browse the web, collecting information from web pages to create a searchable index. Crawlers are important for SEO because they determine the relevance and quality of content, which affects a website's search engine ranking. They follow links from one page to another, finding content to index. Understanding and optimizing a website for these crawlers is crucial for effective SEO.