"terra rara" meaning in Italian

{
  "forms": [
    {
      "form": "terre rare",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "f"
      },
      "expansion": "terra rara f (plural terre rare)",
      "name": "it-noun"
    }
  ],
  "lang": "Italian",
  "lang_code": "it",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "Italian entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "it",
          "name": "Chemistry",
          "orig": "it:Chemistry",
          "parents": [
            "Sciences",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "rare earth element"
      ],
      "id": "en-terra_rara-it-noun-rMmv2c9j",
      "links": [
        [
          "chemistry",
          "chemistry"
        ],
        [
          "rare earth element",
          "rare earth element"
        ]
      ],
      "raw_glosses": [
        "(chemistry) rare earth element"
      ],
      "tags": [
        "feminine"
      ],
      "topics": [
        "chemistry",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "terra rara"
}

{
  "forms": [
    {
      "form": "terre rare",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "f"
      },
      "expansion": "terra rara f (plural terre rare)",
      "name": "it-noun"
    }
  ],
  "lang": "Italian",
  "lang_code": "it",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "Italian countable nouns",
        "Italian entries with incorrect language header",
        "Italian feminine nouns",
        "Italian lemmas",
        "Italian multiword terms",
        "Italian nouns",
        "it:Chemistry"
      ],
      "glosses": [
        "rare earth element"
      ],
      "links": [
        [
          "chemistry",
          "chemistry"
        ],
        [
          "rare earth element",
          "rare earth element"
        ]
      ],
      "raw_glosses": [
        "(chemistry) rare earth element"
      ],
      "tags": [
        "feminine"
      ],
      "topics": [
        "chemistry",
        "natural-sciences",
        "physical-sciences"
      ]
    }
  ],
  "word": "terra rara"
}