Sitemap
A sitemap is a machine-readable list of every address on a website that belongs in search. It tells search engines what exists and when it last changed — but it replaces not one argument for why something deserves to be indexed.
Technically it is an XML file, often split by content type and language and discoverable through a line in robots.txt. It should not be maintained by hand: a decent system generates it itself, leaves drafts out and adds new pages on its own.
It helps most where internal linking is weak — freshly published pages, deeply nested content, multilingual sites with their own address per language. A search engine finds a lot without one, but later.
What a sitemap does not do: it guarantees neither crawling nor indexing nor ranking. It is an offer, not a claim — and an entry in it does not make a weak page strong.