A lien is a recorded security interest in the property of another. The most common types of liens are those on real property and automobiles: i.e., When you take out a mortgage, the lender has a security interest in the house itself -- if you fall behind and fail to pay, they can foreclose and sell the property to recoup their losses. Similarly, if you purchase an automobile and default on payments, your lender can reposses the car to satisfy your debt.
Occasionally, specific items of one's personal property can be subject to a lien to ensure payment of debt.
However, in Oregon a landlord CANNOT enforce a lien against a tenant's household goods to ensure receipt of rent payments or the like, unless the lien was already in place and perfected prior to 1973.
So if a landlord tries to intimidate you by threatening to place a lien on your new furniture or home entertainment system, they are likely threatening to act outside the letter of the law.