A text block is a customizable body of text. It can be found under the "Generic Items" tab when editing your website.
A text block can be directly edited by clicking on it. You can use the black bar on the top of the text block for formatting. Type directly into the box. You can delete the dummy text by selecting it and using the backspace/delete button on your keyboard.
When hovering over the text block with your mouse, click on the pencil icon in the top left corner to access various settings available to customize the object.