Legislation can change over time. For example, sections of an Act may be amended (changed) or repealed (removed).
Some sources provide unamended legislation, which is the law as originally passed (or 'enacted').
Some sources provide amended (or 'consolidated') legislation, which is the law currently in force including any changes made.
Primary legislation means an Act of Parliament (also known as a Statute). They create new law or change existing law.
An Act is a Bill that has been approved by the House of Commons and the House of Lords, then received Royal Assent (agreement from the reigning monarch).
You can find more about Acts on the UK Parliament website.
The most common type of secondary legislation is Statutory Instruments (or 'SI'), also known as delegated legislation. SI are Regulations, Rules or Orders made under the authority of an Act of Parliament.
The function of SI is to support Acts and they do this in three main ways: 1) SI fill in the detail of how an Act will work in practice; 2) SI make changes to Acts and 3) some SI are Commencement Orders, which bring an Act (or section of an Act) into force as law.
You can find more about delegated legislation on the UK Parliament website.