Magento 2 Add a note under a form field using ui components

In php code:

$fieldset->addField(
    'url',
    'text',
    [
        'name' => 'url',
        'label' => __('URL'),
        'title' => __('URL'),
        'required' => false,
        'note' => 'Can be an absolute or relative path.'
    ]
);

In Ui Form:

<item name="notice" xsi:type="string" translate="true">

example:

<field name="sku">
    <argument name="data" xsi:type="array">
        <item name="config" xsi:type="array">
            <item name="label" xsi:type="string" translate="true">Sku</item>
            <item name="visible" xsi:type="boolean">true</item>
            <item name="dataType" xsi:type="string">text</item>
            <item name="formElement" xsi:type="string">input</item>
            <item name="source" xsi:type="string">product_information</item>
            <item name="dataScope" xsi:type="string">sku</item>
            <item name="disabled" xsi:type="boolean">true</item>
            <item name="notice" xsi:type="string" translate="true">Auto Created</item>
            <item name="validation" xsi:type="array">
                <item name="required-entry" xsi:type="boolean">true</item>
            </item>
        </item>
    </argument>
</field>

More Magento 2 Document, Please View https://www.tonyblog.cn/category/6