The type is an optional attribute for a backend configuration form field.
An example for the core PayPal module’s option «Sales» → «Payment Methods» → «PayPal Payment Solutions» → «Required PayPal Settings» → «Express Checkout» → «API Certificate»:

The type attribute is optional and has the default value text:
Magento 2 supports the following standard values for the type attribute: Magento 2 standard form field types
A type value could also be a full class name.
Magento 2 creates a field instance based on its type: How does \Magento\Framework\Data\Form\Element\Fieldset::addField() work?