A check-box is a component that can be in either an "on" (true) or "off" (false) state. Clicking on a check-box changes its state from "on" to "off," or from "off" to "on."
After selecting to create a "Checkbox" data type, simply enter a name for the data type in the window that appears and click "Save."
The new "checkbox" data type will then be added to the "List of Available Datatypes," so that it can be used to create fields within your customized forms, by simply selecting the data type from the drop-down menu in the section used to add custom fields.
Here is an example of the output of a form including a number of "checkbox" fields: