A lien is a claim that is on a property until the debt on that property has been paid off. A mortgage on your property is considered a lien. Any debts you may have, like a medical bill, could attach any properties you own and must be paid off at the sale of your property. If you have a mortgage on your property that mortgage is considered a lien. In short a lien is a debt that you owe to somebody that must be paid before you are able to sell your property.