What this means is that the federal prosecutors do not have sufficient evidence to pursue any one particular person. Therefore, the subpoena that might have been used on Mary-Kate Olsen to compel her to testify is no longer valid. Apparently, the subpoena was reliant on the federal prosecutors pursing the case. If they do not pursue the case then the subpoena cannot be enforced.
However, deciding not to pursue the case at this point in time does not mean that it could not be pursued at some later time should evidence become available that a crime has been committed.