What means «type» attribute for a backend configuration's form field?

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?