What does it mean to put a lien on someone's condo?
A lien is a legal instrument which prevents the property it is attached to from being transferred, sold, refinanced, and so forth by the owners until the debt pertaining to the judgment lien is paid in full or satisfied.
A "perfected lien" against real property becomes a secured debt and in some states under specific conditions it allows the lien holder to petition the court for the forced sale of the named property.
When a lien is filed on a condominium, the lien is based on a standing agreement between the owner and the association that the owner will pay monies to the association which have not been paid. There is no 'judgement' involved.
You can find the details of this agreement in the association's governing documents.
Once a lien is filed, it not only clouds title to the property, as above, but can appear on the owner's credit report.