A thorough explanation would be too much for a post or even a topic. As can be seen here https://www.w3.org/TR/2011/REC-SVG11-20110816/intro.html#NamespaceAndDTDIdentifiers
the “xmlns” is “XML Namespace” which is needed to use the correct “DTD” - “Doctype Declaration”
Kind of like the “rules” about what elements are allowed, what attributes elements can have, what child tags elements can have, what parent tags elements can have, what they must have, etc.
In other words, it’s kind of like a way of telling browsers “parse the mark-up according to these rules”
How, or even If, browsers actually parse according to the DTD varies, but having a definition to go by is a good thing.