"terres rares" meaning in Français

{
  "anagrams": [
    {
      "word": "reresserrât"
    },
    {
      "word": "reterrasser"
    }
  ],
  "categories": [
    {
      "kind": "other",
      "name": "Formes de locutions nominales en français",
      "parents": [],
      "source": "w"
    },
    {
      "kind": "other",
      "name": "Français",
      "orig": "français",
      "parents": [],
      "source": "w"
    }
  ],
  "forms": [
    {
      "form": "terre rare",
      "tags": [
        "singular"
      ]
    }
  ],
  "lang": "Français",
  "lang_code": "fr",
  "notes": [
    "Le pluriel est plus fréquemment utilisé que le singulier."
  ],
  "pos": "noun",
  "pos_title": "Forme de locution nominale",
  "senses": [
    {
      "form_of": [
        {
          "word": "terre rare"
        }
      ],
      "glosses": [
        "Pluriel de terre rare."
      ],
      "id": "fr-terres_rares-fr-noun-bFhlScqJ"
    }
  ],
  "sounds": [
    {
      "ipa": "\\tɛʁ ʁɑʁ\\"
    }
  ],
  "tags": [
    "feminine",
    "form-of"
  ],
  "word": "terres rares"
}

{
  "anagrams": [
    {
      "word": "reresserrât"
    },
    {
      "word": "reterrasser"
    }
  ],
  "categories": [
    "Formes de locutions nominales en français",
    "français"
  ],
  "forms": [
    {
      "form": "terre rare",
      "tags": [
        "singular"
      ]
    }
  ],
  "lang": "Français",
  "lang_code": "fr",
  "notes": [
    "Le pluriel est plus fréquemment utilisé que le singulier."
  ],
  "pos": "noun",
  "pos_title": "Forme de locution nominale",
  "senses": [
    {
      "form_of": [
        {
          "word": "terre rare"
        }
      ],
      "glosses": [
        "Pluriel de terre rare."
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "\\tɛʁ ʁɑʁ\\"
    }
  ],
  "tags": [
    "feminine",
    "form-of"
  ],
  "word": "terres rares"
}