"серная" meaning in Russian

{
  "forms": [
    {
      "form": "се́рная",
      "tags": [
        "canonical"
      ]
    },
    {
      "form": "sérnaja",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ru",
        "2": "adjective form",
        "head": "се́рная"
      },
      "expansion": "се́рная • (sérnaja)",
      "name": "head"
    }
  ],
  "lang": "Russian",
  "lang_code": "ru",
  "pos": "adj",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Russian entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        }
      ],
      "form_of": [
        {
          "extra": "sérnyj",
          "word": "се́рный"
        }
      ],
      "glosses": [
        "nominative feminine singular of се́рный (sérnyj)"
      ],
      "id": "en-серная-ru-adj-LIb~e2TH",
      "links": [
        [
          "се́рный",
          "серный#Russian"
        ]
      ],
      "tags": [
        "feminine",
        "form-of",
        "nominative",
        "singular"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[ˈsʲernəjə]"
    }
  ],
  "word": "серная"
}

{
  "forms": [
    {
      "form": "се́рная",
      "tags": [
        "canonical"
      ]
    },
    {
      "form": "sérnaja",
      "tags": [
        "romanization"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "ru",
        "2": "adjective form",
        "head": "се́рная"
      },
      "expansion": "се́рная • (sérnaja)",
      "name": "head"
    }
  ],
  "lang": "Russian",
  "lang_code": "ru",
  "pos": "adj",
  "senses": [
    {
      "categories": [
        "Pages with 1 entry",
        "Pages with entries",
        "Russian 3-syllable words",
        "Russian adjective forms",
        "Russian entries with incorrect language header",
        "Russian non-lemma forms",
        "Russian terms with IPA pronunciation"
      ],
      "form_of": [
        {
          "extra": "sérnyj",
          "word": "се́рный"
        }
      ],
      "glosses": [
        "nominative feminine singular of се́рный (sérnyj)"
      ],
      "links": [
        [
          "се́рный",
          "серный#Russian"
        ]
      ],
      "tags": [
        "feminine",
        "form-of",
        "nominative",
        "singular"
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "[ˈsʲernəjə]"
    }
  ],
  "word": "серная"
}