"un à un" meaning in French

{
  "head_templates": [
    {
      "args": {},
      "expansion": "un à un",
      "name": "fr-adv"
    }
  ],
  "lang": "French",
  "lang_code": "fr",
  "pos": "adv",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "French entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "fr",
          "name": "One",
          "orig": "fr:One",
          "parents": [
            "Numbers",
            "All topics",
            "Terms by semantic function",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "one at a time; one by one"
      ],
      "id": "en-un_à_un-fr-adv-fRKf2zI9",
      "links": [
        [
          "one at a time",
          "one at a time"
        ],
        [
          "one by one",
          "one by one"
        ]
      ],
      "synonyms": [
        {
          "word": "un par un"
        }
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/œ̃.n‿a œ̃/"
    }
  ],
  "word": "un à un"
}

{
  "head_templates": [
    {
      "args": {},
      "expansion": "un à un",
      "name": "fr-adv"
    }
  ],
  "lang": "French",
  "lang_code": "fr",
  "pos": "adv",
  "senses": [
    {
      "categories": [
        "French adverbs",
        "French entries with incorrect language header",
        "French lemmas",
        "French multiword terms",
        "French terms with IPA pronunciation",
        "fr:One"
      ],
      "glosses": [
        "one at a time; one by one"
      ],
      "links": [
        [
          "one at a time",
          "one at a time"
        ],
        [
          "one by one",
          "one by one"
        ]
      ],
      "synonyms": [
        {
          "word": "un par un"
        }
      ]
    }
  ],
  "sounds": [
    {
      "ipa": "/œ̃.n‿a œ̃/"
    }
  ],
  "word": "un à un"
}