A mechanic's lien is a legal claim registered against a property by someone who supplied labour or materials to improve it and was not paid.
It attaches to the property rather than to the customer, which is what gives it force: the property generally cannot be sold or refinanced cleanly while it stands.
Why it is the strongest collection tool a contractor has
Ordinary collections rely on persuading someone to pay. A lien does not. It creates a problem the owner has to resolve, and it frequently resolves the debt without proceeding further.
The deadlines are the whole of it
Lien rights are governed by state law and every state sets time limits. They are typically measured from the last date work was performed or materials supplied, and they are short — often a matter of weeks or a few months.
Miss the deadline and the right is gone permanently, regardless of the merits.
Preliminary notice
Many states require a notice served at or near the start of work as a precondition of any later lien. A contractor who does not serve it may have no rights at all, however clear the debt.
This is why lien handling belongs at the start of a job rather than at the point of non-payment.
Waivers
Customers and lenders commonly require a waiver in exchange for payment, giving up lien rights for the work covered.
The distinction that matters is between a waiver conditional on the payment clearing and one that is unconditional. Signing an unconditional waiver before the money has arrived gives up the right while the debt still exists.
Getting it right
The rules are state-specific, procedural, and unforgiving of error. This is an area to run with a construction attorney in the state rather than from a general description.