"it's on" meaning in English

{
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "phrase"
      },
      "expansion": "it's on",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "phrase",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "English pro-sentences",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        }
      ],
      "derived": [
        {
          "word": "on like Donkey Kong"
        }
      ],
      "examples": [
        {
          "ref": "1959: Girls Town (film)",
          "text": "[A man from a competing gang throws a drink in Fred's face, just before a fight begins.]\nFred Alger (Mel Tormé): It's on!"
        }
      ],
      "glosses": [
        "A conflict or competition is starting."
      ],
      "id": "en-it's_on-en-phrase-xDE1aXeG",
      "links": [
        [
          "conflict",
          "conflict"
        ],
        [
          "competition",
          "competition"
        ]
      ],
      "raw_glosses": [
        "(colloquial) A conflict or competition is starting."
      ],
      "related": [
        {
          "word": "bring it on"
        }
      ],
      "tags": [
        "colloquial"
      ]
    }
  ],
  "word": "it's on"
}

{
  "derived": [
    {
      "word": "on like Donkey Kong"
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "en",
        "2": "phrase"
      },
      "expansion": "it's on",
      "name": "head"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "phrase",
  "related": [
    {
      "word": "bring it on"
    }
  ],
  "senses": [
    {
      "categories": [
        "English colloquialisms",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English phrases",
        "English pro-sentences",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "examples": [
        {
          "ref": "1959: Girls Town (film)",
          "text": "[A man from a competing gang throws a drink in Fred's face, just before a fight begins.]\nFred Alger (Mel Tormé): It's on!"
        }
      ],
      "glosses": [
        "A conflict or competition is starting."
      ],
      "links": [
        [
          "conflict",
          "conflict"
        ],
        [
          "competition",
          "competition"
        ]
      ],
      "raw_glosses": [
        "(colloquial) A conflict or competition is starting."
      ],
      "tags": [
        "colloquial"
      ]
    }
  ],
  "word": "it's on"
}