Part of the work of Parliament is to make laws. These are called Acts of Parliament. Usually the House of Commons and the House of Lords both debate proposals for new laws called Bills. If both Houses vote for the proposals then the Bill is ready to become an Act. It can only be described as an Act when it has received Royal Assent from the monarch.
Further information on this subject can be found from the following links.
About Parliament: Acts
Acts of Parliament from legislation.gov.uk