I'm looking at this example of a dynamic form from ant design, and struggling to figure out exactly what's going on:

import { Form, Input, Button, Space } from 'antd';
import { MinusCircleOutlined, PlusOutlined } from '@ant-design/icons';

const Demo = () => {
  const onFinish = values => {
    console.log('Received values of form:', values);
  };

  return (
    <Form name="dynamic_form_nest_item" onFinish={onFinish} autoComplete="off">
      <Form.List name="users">
           {(fields, { add, remove }) => ( // ### THIS LINE
          <>
            {fields.map(({ key, name, ...restField }) => (
              <Space key={key} style={{ display: 'flex', marginBottom: 8 }} align="baseline">
                <Form.Item
                  {...restField}
                  name={[name, 'first']}
                  rules={[{ required: true, message: 'Missing first name' }]}
                >
                  <Input placeholder="First Name" />
                </Form.Item>
                <Form.Item
                  {...restField}
                  name={[name, 'last']}
                  rules={[{ required: true, message: 'Missing last name' }]}
                >
                  <Input placeholder="Last Name" />
                </Form.Item>
                <MinusCircleOutlined onClick={() => remove(name)} />
              </Space>
            ))}
            <Form.Item>
              <Button type="dashed" onClick={() => add()} block icon={<PlusOutlined />}>
                Add field
              </Button>
            </Form.Item>
          </>
        )}
      </Form.List>
      <Form.Item>
        <Button type="primary" htmlType="submit">
          Submit
        </Button>
      </Form.Item>
    </Form>
  );
};

ReactDOM.render(<Demo />, mountNode);

I can't quite figure out what {(fields, { add, remove }) => ( is doing, however fields, add, and remove all play critical roles in the functionality. For example, calling "add" by clicking the button adds a new entry field, and it's not immediately clear why.

What I'm trying to solve: I'd like to populate a form like this with my own, dynamic data (from an API call) on page load, allow the user to add and remove rows (saved in state on each change), and have all of those rows be bound to some condition (e.g. spit an error if the rows don't add up to 100%)