This is a HL7 FHIR server, and as you appear to be accessing this page from a web browser you are seeing a HTML version of the requested resource(s). You can also access this URL from a FHIR client as a ReSTful API call. For more details please see the HL7 FHIR specification.
{
  "resourceType": "NamingSystem",
  "name": "NHS Number",
  "status": "draft",
  "kind": "identifier",
  "date": "2015-05-01",
  "publisher": "NHS Digital",
  "contact": [
    {
      "name": "Interoperability Team",
      "telecom": [
        {
          "system": "email",
          "value": "interoperabilityteam@nhs.net",
          "use": "work"
        }
      ]
    }
  ],
  "responsible": "NHS Digital",
  "description": "NHS Number allocated to the patient.",
  "usage": "A unique patient identifier for everyone registered with the NHS.",
  "uniqueId": [
    {
      "type": "uri",
      "value": "https://fhir-test.hl7.org.uk/Id/nhs-number",
      "preferred": true
    },
    {
      "type": "oid",
      "value": "2.16.840.1.113883.2.1.4.1",
      "preferred": false
    }
  ]
}