Liens are used in the world of finance, and also in the justice system. It is the claim against a property by a creditor or a court. A lien is removed once the debt is paid or a court ruling is satisfied.
Lien searches are due diligence involving a thorough search to see if there are any claims against the property of a business or an individual. Main sources of information in a lien search are the Provincial Enquiry Centres here in Canada, and County Clerk's and Secretary of State's offices in the USA.
It goes without saying that a lien search cannot be done here in Canada on a boat (or any other piece of property) that is from the United States or any other foreign country. It never ceases to amaze me, the number of people that still think it can be.