"key to the door" meaning in English

{
  "etymology_text": "A reference to the traditional age of the majority.",
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "key to the door",
      "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"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Bingo",
          "orig": "en:Bingo",
          "parents": [
            "Games",
            "Recreation",
            "Human activity",
            "Human behaviour",
            "Human",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Twenty-one."
      ],
      "id": "en-key_to_the_door-en-noun-ttdKmGrL",
      "links": [
        [
          "bingo",
          "bingo"
        ],
        [
          "Twenty-one",
          "twenty-one"
        ]
      ],
      "raw_glosses": [
        "(bingo) Twenty-one."
      ],
      "topics": [
        "bingo",
        "games"
      ]
    }
  ],
  "word": "key to the door"
}

{
  "etymology_text": "A reference to the traditional age of the majority.",
  "head_templates": [
    {
      "args": {
        "1": "?"
      },
      "expansion": "key to the door",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English nouns with unknown or uncertain plurals",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Bingo"
      ],
      "glosses": [
        "Twenty-one."
      ],
      "links": [
        [
          "bingo",
          "bingo"
        ],
        [
          "Twenty-one",
          "twenty-one"
        ]
      ],
      "raw_glosses": [
        "(bingo) Twenty-one."
      ],
      "topics": [
        "bingo",
        "games"
      ]
    }
  ],
  "word": "key to the door"
}