Crawlability is how easily an automated agent can reach, read and understand a site's content without executing complex client-side behaviour.
It governs two audiences that are converging. Search engines need to reach content to index it; answer engines and AI crawlers need to reach it to cite it. Both are far less tolerant of client-side rendering than a browser is.
The practical checklist is unglamorous: real HTML in the initial response, semantic headings, links as anchors, a sitemap, permissive robots rules for the agents you want, and structured data describing what each page is.
For internal site search the same property matters: content that a crawler cannot reach is content the site's own index will miss.