A Money Bill is a Bill that deals with national taxation, public money or loans and their management. It is only considered to be a Money Bill if the Speaker of the House of Commons agrees that it is. The Speaker signs a certificate to indicate this. The most important Money Bills are those that deal with taxation bills or the Consolidated Fund Bills that formally vote money to the government. Elements of the Parliament Acts apply to Money Bills and ensure that they pass through the Lords within a month of being agreed by the Commons. As the House of Lords has no power over Money Bills they can gain Royal Assent without the Lords' approval.
Further information on this subject can be found from the following links.
Public Bills before Parliament
Background information - The Parliament Acts
Background information - An Introduction to Parliament : Royal Assent