In this context the specific thing they are is certificates from the Web PKI. A PKI (Public Key Infrastructure) is an arrangement with Relying Parties (in this case, basically everybody), CAs (Certificate Authorities - in this case a mix of companies, not-for-profits, government and so on entities around the world) and Subscribers. The Subscriber says to a CA "I want you to certify that I'm some.website.example" and the CA issues them an X.509 certificate, which the Relying Parties trust to prove that this really is some.website.example. The Relying Parties (indirectly as we'll see shortly) ensure they trust only CAs who will do this name certifying job well. This uses Public Key encryption, which is a mathematical technology where you pick two related huge numbers, one public key (revealed to anyone who wants it) and one private (known only to you) and then you can prove you know the private key by performing arithmetic that anyone with the public key can verify is correct, and yet they could not perform that arithmetic without your private key.

It is called the Web PKI because although this secures most of the Internet, the billions of Relying Parties are represented in practice almost solely by a handful of Trust Stores who mostly make Web Browsers. Specifically, Mozilla, Google, Microsoft and Apple.

The Web PKI requires that the certificates are not only X.509 but specifically they obey PKIX, RFC 5280 which explains how X.509 (a standard from the X.500 directory system, a directory which in reality never ended up existing) can be used for the Internet (which very much did end up existing) via "Alternative Names". When your modern certificates have a "Subject Alternative Name" the word Alternative there means alternative to the X.500 naming scheme, which is irrelevant to us, specifically the Internet's two alternatives, an ipAddress (4 bytes or 16 bytes forming either an IPv4 or IPv6 address) or a dnsName (a subset of ASCII characters, punctuated with but never ending in a dot)

Edited: Correct s/Server/Subject/ in expansion of SAN acronym