In this case [%CurrentObject%] is a token representing the id of the parent entity. Let's assume this is A.
Your datagrid for B objects is inside of a dataview containing an A object, the “CurrentObject” refers to this containing A object.
The statement [A_B=[%CurrentObject%]] could be translated to:
Retrieve all B objects for which its association A_B is equal to the id of the object A from the containing dataview.