Every visit starts with a phone-book lookup: the browser asks a resolver, the resolver asks the authoritative nameserver, the answer — an IP — is cached along the way for the record's TTL. It is invisible until it is not: 'site down' is very often 'DNS down' or 'DNS propagating'.
The records a business site actually manages: A/AAAA (the server), CNAME (aliases like www), MX (who gets the email), TXT (SPF, DKIM, verification tokens — email deliverability and domain ownership live here). Changing them is instant to save and slow to spread, because every resolver on earth cached the old answer.
Related terms
CDN
Content delivery network — copies of your assets on servers near your visitors, so 'far away' stops meaning 'slow'.
TLS
Transport Layer Security — the encryption layer behind HTTPS that keeps traffic private and proves the site you reached is the site you meant.
Indexing
A search engine adding a page to its searchable database — crawling fetches it, indexing makes it findable; unindexed is invisible.
