Crawlers discover pages by following links, which works — eventually, and only for what is linked well. A sitemap is the explicit inventory: every URL, when it last changed, optionally which alternates exist in other languages. It is a declaration, not a guarantee, but it removes 'we never crawled that page' from the excuse list.
The lastmod date is the field that earns its keep: an honest timestamp tells crawlers to spend their budget on what changed. A sitemap that claims everything was modified today trains crawlers to ignore it — the rare case where lying to a robot actively hurts you.
Related terms
robots.txt
A plain-text file at the site root telling crawlers what they may fetch — an instruction to machines, not a lock on a door.
Indexing
A search engine adding a page to its searchable database — crawling fetches it, indexing makes it findable; unindexed is invisible.
Canonical URL
A tag naming the one 'real' address for content reachable at several URLs — so search engines rank the copy you meant, once.
The bench this belongs to
Web performanceA rebuild that loads on a phone on mobile data, in the country your customers are actually in. Core Web Vitals are the measurement, revenue is the point.
