"flat back four" meaning in English

{
  "forms": [
    {
      "form": "flat back fours",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "head": "flat back four"
      },
      "expansion": "flat back four (plural flat back fours)",
      "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": "topical",
          "langcode": "en",
          "name": "Football (soccer)",
          "orig": "en:Football (soccer)",
          "parents": [
            "Football",
            "Ball games",
            "Sports",
            "Human activity",
            "Human behaviour",
            "Human",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "collectively, a set of defenders who stay on the same level on the pitch."
      ],
      "id": "en-flat_back_four-en-noun-3kapIQye",
      "links": [
        [
          "soccer",
          "soccer"
        ],
        [
          "defender",
          "defender"
        ]
      ],
      "raw_glosses": [
        "(soccer) collectively, a set of defenders who stay on the same level on the pitch."
      ],
      "related": [
        {
          "word": "wing back"
        }
      ],
      "topics": [
        "ball-games",
        "games",
        "hobbies",
        "lifestyle",
        "soccer",
        "sports"
      ]
    }
  ],
  "word": "flat back four"
}

{
  "forms": [
    {
      "form": "flat back fours",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "head": "flat back four"
      },
      "expansion": "flat back four (plural flat back fours)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "related": [
    {
      "word": "wing back"
    }
  ],
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "en:Football (soccer)"
      ],
      "glosses": [
        "collectively, a set of defenders who stay on the same level on the pitch."
      ],
      "links": [
        [
          "soccer",
          "soccer"
        ],
        [
          "defender",
          "defender"
        ]
      ],
      "raw_glosses": [
        "(soccer) collectively, a set of defenders who stay on the same level on the pitch."
      ],
      "topics": [
        "ball-games",
        "games",
        "hobbies",
        "lifestyle",
        "soccer",
        "sports"
      ]
    }
  ],
  "word": "flat back four"
}