That's when the LLM can respond with multiple functions it wants you to call at once. You might send it:

    Location and population of Paris, France

A parallel function calling LLM could return:

    {
      "role": "assistant",
      "content": "",
      "tool_calls": [
        {
          "function": {
            "name": "get_city_coordinates",
            "arguments": "{\"city\": \"Paris\"}"
          }
        }, {
          "function": {
            "name": "get_city_population",
            "arguments": "{\"city\": \"Paris\"}"
          }
        }
      ]
    }

Indicating that you should execute both of those functions and return the results to the LLM as part of the next prompt.