A lien is a legal claim a creditor records against a property as security for a debt, and it attaches to the property itself rather than to you personally. That distinction is the whole point: because the claim rides with the title, the debt usually has to be paid off before ownership can change hands cleanly.
Liens come in two flavors. A voluntary lien is one you agree to, like the mortgage you sign at closing. An involuntary lien is one a creditor places without your consent, like a contractor's mechanic's lien for unpaid work, a tax lien from the county, or a judgment lien from a lawsuit you lost.
In practice
Say you go to sell a rental you bought years ago for $260,000, and a buyer offers $410,000. You still owe $180,000 on the mortgage, which is itself a lien. The title search then turns up two more: a $9,500 mechanic's lien a roofer filed when an old invoice slipped through the cracks, and a $1,200 county lien for a missed sewer assessment. None of these vanish on their own.
At the closing table the math is mechanical. From the $410,000 sale price the settlement agent pays off the $180,000 mortgage, the $9,500 roofer lien, and the $1,200 sewer lien, a total of $190,700 in liens cleared before a dollar reaches you. The buyer gets clear title, and you walk away with the remainder minus your other selling costs. Had you not addressed the smaller liens earlier, they would have surfaced here anyway and slowed the deal.
Why it matters to a small landlord
Liens bite at the two moments a property changes state: buying and selling. When you buy, the title search exists to flush out claims like these before they become yours, which is one reason a lien check sits near the top of any first rental purchase and why lenders insist on it. When you hold, the lesson is quieter: every unpaid contractor invoice and every skipped property tax bill is a lien waiting to be filed. Keeping vendor payments and tax deadlines current is cheaper than clearing a recorded claim later, which is exactly the kind of thing a month-by-month landlord checklist is built to catch.
The other defense is documentation. A roofer who is paid on time and marked paid in your records has no grounds to file, and you have proof if they try.
Liens rarely come up alone. The due diligence period is when you go looking for them, a clean title insurance policy is what protects you if one slips past the search, and a warranty deed is the seller's promise that the title they are handing you is free of undisclosed claims. Read in that order, the three explain how a buyer ends up with title nobody else can touch.