Gender Recognition Bill: What is a Section 35 Order?

The Secretary of State for Scotland Alister Jack has now confirmed the UK government will issue a Section 35 order under the terms of the 1998 Scotland Act, to prevent the Scottish bill from receiving royal assent and becoming law. But what is Section 35 of the 1998 Scotland Act and how does it make a provision for the UK goverment to veto a bill that has been passed by the Scottish government?