The Multi-Select Box attribute allows you to select multiple values by using a drag and drop interface. It is similar to checkboxes as it allows you to choose multiple values but its benefit is the ability to more easily distinguish which values are chosen and which ones aren't. You can also re-order the selected values if desired.
Multi-Select Box using data from an App API call
The Multi-Select Box using data from an App API call attribute allows you to pull in data from another app by using an API call that is set up when the attribute is created. This info that is retrieved by the API call can then be used in your content.
An example scenario could be to associate a blog category with a page to output recent posts from that category on the page.
View the documentation about the App API call for more information.