SelectionMiniToolBar is a floating toolbar that appears next to the current position after making a selection with the mouse in the document. It contains some useful commands for formatting text.
As RadRichTextBox loads the default SelectionMiniToolBar using MEF, it can be completely replaced with a custom one. This can be done as easy as with the other RadRichTextBox dialogs by implementing the interface ISelectionMiniToolBar and marking the class that implements it with the CustomSelectionMiniToolBarAttribute attribute. The technique was also described in greater details in this blog post.
And of course, for those of you who don't need additional UI pop-ups, these can be disabled by setting the IsSelectionMiniToolBarEnabled property of RadRichTextBox to False.