A Text block is used to add text or images on a module. You can use this block to add contact details for the module teaching team.
This block is very flexible and can be used for a variety of other purposes. It uses the standard Moodle text editor for formatting text, adding images or creating links. You can switch to code view to enter valid HTML markup. This enables embedding of video, audio and other files which can add unique elements to a module page. You can add multiple Text blocks to your module page
In Moodle 3.9, Text block is called HTML block.