What it means
Automated programs called crawlers or bots visit URLs to read text, images, and other page resources. Search engines discover URLs through links, sitemaps, and pages they already know.
Example
You publish an article and link to it from your blog page. A search engine can discover that link and visit the article to read its content.
Keep in mind
Crawling and indexing are separate steps. A search engine can visit a page without adding it to its searchable index.