This main window button deletes one or more attribute value - according to the current selection in the attribute list. ATTENTION: LEX cannot undo a delete operation in the directory. So be careful if you answer yes to the regarding warning message.
To remove an attribute, you could also press the DELETE key on your keyboard or use the menu option Edit - Remove Attribute when the focus is set to the attribute list panel.
If you selected a single value attribute line in the attribute list, the attribute will be deleted. If you selected one line of a multi valued attribute, then only the according array value will be removed from the attribute:
If you want to remove the entire attribute, just select the header line and use the Remove attribute Button :
By the way: If you see the Remove attribute button disabled and gray, then your LEX is probably in Read Only mode.