He has the power to make rulings on questions of evidence and issue all orders, mandates and writs authorized by the Senate. However, a senator may ask that to vote on any ruling by the Chief Justice. Any Rehnquist ruling can be overturned by a simple majority. Rehnquist will direct all proceedings during the trial. All questions from senators must be submitted to him in writing, unless the rules created by the Senate for this trial specify otherwise.
Senators are ultimately both judges and jurors.
They rule on the facts and the law. The decide if the evidence shows that Clinton committed perjury and obstruction of justice. Rehnquist is the presiding officer, which means he merely directs the proceedings. He will offer initial rulings on evidence, but he can be overturned by a majority of senators. The Senate is not bound by federal rules of evidence.