"known unknown" meaning in English

{
  "etymology_text": "Popularized by Donald Rumsfeld in 2002, though used much earlier.",
  "forms": [
    {
      "form": "known unknowns",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "known unknown (plural known unknowns)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "An uncertainty of known magnitude, consequence, structure, and probability characteristics; something that we know that we don't know."
      ],
      "id": "en-known_unknown-en-noun-Uvd9~Q1r",
      "links": [
        [
          "uncertainty",
          "uncertainty"
        ]
      ],
      "related": [
        {
          "word": "known known"
        },
        {
          "word": "unknown unknown"
        },
        {
          "word": "unknown known"
        }
      ],
      "wikipedia": [
        "Donald Rumsfeld",
        "There are known knowns"
      ]
    }
  ],
  "word": "known unknown"
}

{
  "etymology_text": "Popularized by Donald Rumsfeld in 2002, though used much earlier.",
  "forms": [
    {
      "form": "known unknowns",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "known unknown (plural known unknowns)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "known known"
    },
    {
      "word": "unknown unknown"
    },
    {
      "word": "unknown known"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns"
      ],
      "glosses": [
        "An uncertainty of known magnitude, consequence, structure, and probability characteristics; something that we know that we don't know."
      ],
      "links": [
        [
          "uncertainty",
          "uncertainty"
        ]
      ],
      "wikipedia": [
        "Donald Rumsfeld",
        "There are known knowns"
      ]
    }
  ],
  "word": "known unknown"
}