Indexability is whether a page, once crawled, is eligible to be stored in a search index and surfaced in results. A page can be crawlable but not indexable if it carries a noindex tag, is canonicalized to another URL, or is otherwise marked to be excluded.
For brands, indexability matters for AI visibility because several major AI surfaces ground their answers in a search index. Google's AI Overviews, AI Mode, and Gemini draw on Google's index, so a page that is not indexed there is far less likely to appear in those AI answers. Confirming that your important pages are both crawlable and indexable keeps them eligible across search and the AI surfaces built on top of it.