An accountant's lien is a legal right belonging to accountants that helps them ensure that they get paid. The way that this right works is that the accountant does not have to relinquish the property of their client until he or she receives payment for services rendered. This property usually includes documents.
Accountants' liens exist because it would be very easy for clients to rip off accountants if they did not. For example, if an accountant returned a completed tax form to a client, the client could simply take the tax form and put it in the mail without ever having to pay the accountant. This would essentially rob the accountant of the money that he/she earned doing the business. So, accountants' liens serve as a type of insurance for accountants to make sure that they will be paid.