The structure of the XML that is used to define a SCORM manifest is defined through a series of XSD (XML schema definition) documents. XML requires that each of these XSDs be associated with a unique identifier that points to its "namespace". That is what all of the gobiligook at the top of the manifest file is about.
In the screenshot below, you can see that the manifest contains some elements that are defined in the "xmlns:adlcp" (ADL Content Packaging) namespace. This namespace is given the identifier "http://www.adlnet.org/xsd/adlcp_v1p3". Down below, in the schemaLocation attribute, you can find that the elements in this namespace are defined in the XSD file named "adlcp_v1p3.xsd" (which should be included with your SCORM package).
People often ask "But when I go to 'http://www.adlnet.org/xsd/adlcp_v1p3', there's nothing there. Something must be wrong". This is a common misunderstanding. The value "http://www.adlnet.org/xsd/adlcp_v1p3" is a URI, not a URL.
URI = Uniform Resource Identifier
URL = Uniform Resource Locator
A URI is simply a globally unique identifier. A URL is one type of URI. A URL is always a URI, but a URL is not necessarily a URL.
When defining URIs is it a common practice to use a value that contains a domain name that you control. This is a convenient way of ensuring that your identifier doesn't conflict with somebody else's (provided everybody plays by the same rules). This syntax is not meant to imply that the resource actually exists at this URL location (although there is nothing preventing you from publishing it there).