What is the use of "mapping" node in di.xml in Magento 2?

When you check Magento\Payment\Model\Checks\SpecificationFactory of the construt

public function __construct(\Magento\Payment\Model\Checks\CompositeFactory $compositeFactory, array $mapping)
    {
        $this->compositeFactory = $compositeFactory;
        $this->mapping = $mapping;
    }

Second argument array $mapping in the array you will get all your argument which you defined in di.xml

EDIT

from the construct $this->mapping you will find all of the objects in the array which you defined in di.xml.

For example If you set <argument name="mappingtest" xsi:type="array"> then you need to define construct as

 public function __construct(
\Magento\Payment\Model\Checks\CompositeFactory $compositeFactory, array $mappingytest
) {
    $this->compositeFactory = $compositeFactory;
    $this->mapping = $mapping;
}

Here $mappingytest

The object manager injects the Magento\Core\Model\Url implementation class wherever there is a request for the Magento\Core\Model\UrlInterface in the global scope.

Multiple validation for different modules with filed.

<argument name="mapping" xsi:type="array">
                <item name="country" xsi:type="object">Magento\Payment\Model\Checks\CanUseForCountry</item>
                <item name="currency" xsi:type="object">Magento\Payment\Model\Checks\CanUseForCurrency</item>
                <item name="checkout" xsi:type="object">Magento\Payment\Model\Checks\CanUseCheckout</item>
                <item name="internal" xsi:type="object">Magento\Payment\Model\Checks\CanUseInternal</item>
                <item name="total" xsi:type="object">Magento\Payment\Model\Checks\TotalMinMax</item>
                <item name="zero_total" xsi:type="object">Magento\Payment\Model\Checks\ZeroTotal</item>
            </argument>

Now check SpecificationFactory file for Business logic.

vendor/magento/module-payment/Model/Checks/SpecificationFactory.php

class SpecificationFactory
{
    /**
     * Composite Factory
     *
     * @var \Magento\Payment\Model\Checks\CompositeFactory
     */
    protected $compositeFactory;

    /**
     * @var array
     */
    protected $mapping;

    /**
     * Construct
     *
     * @param \Magento\Payment\Model\Checks\CompositeFactory $compositeFactory
     * @param array $mapping
     */
    public function __construct(\Magento\Payment\Model\Checks\CompositeFactory $compositeFactory, array $mapping)
    {
        $this->compositeFactory = $compositeFactory;
        $this->mapping = $mapping;
    }

    /**
     * Creates new instances of payment method models
     *
     * @param array $data
     * @return Composite
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function create($data)
    {
        $specifications = array_intersect_key($this->mapping, array_flip((array)$data));
        return $this->compositeFactory->create(['list' => $specifications]);
    }
}

It validates the mapping data module wise.

$specifications = array_intersect_key($this->mapping, array_flip((array)$data));