What explanatory notes are, and why Ask The Beagle cites them.
Explanatory notes are plain-language companions to an Act, prepared by the government department responsible for it. They describe what the Act is intended to do and why. But they are commentary: the Act's own words are the law, and the notes are not. legislation.gov.uk describes them among the documents published with legislation:
A variety of documents can be published alongside legislation. A selection of the most common documents are explained below: Explanatory Note(s): Explanatory Notes are documents that typically accompany primary legislation (e.g. an Act or Measure). The text is created by the government department responsible for the subject matter of the Act (or Measure) to explain what the Act sets out to achieve and to make the Act accessible to readers who are not legally qualified. Explanatory Notes were introduced in 1999 and accompany all Public Acts except Appropriation, Consolidated Fund, Finance and Consolidation Acts.
— Understanding Legislation (legislation.gov.uk): Associated documents published with legislation — read in full ↗