An association has a lien on a lot or unit for assessments.  More specifically, an association has a lien for a charge imposed by the association against a lot or unit pursuant to a recorded governing document, and for attorney fees, court costs, late fees, interest and any amount the association is entitled to recover under the declaration or a judicial decision.

A lien is a right that a creditor has in another's property.

In an HOA, the HOA is the creditor.  The right the HOA has is the right to sell the property to satisfy the debt (foreclosure).  But, that right doesn't mature or fully vest until the property owner fails to pay.  Once the owner fails to pay, the association has a right to sell the property at public auction.

A lien is tied to the property itself, and not the owner of the property.  So, if Owner A owns a lot and fails to pay assessments, the HOA has a lien on the lot.  If Owner A conveys the lot to Owner B, the HOA still has a lien against the lot and may foreclose its lien and take ownership.

The lien is

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Statutes and Cases:

(For reference: 57-8 is the Condo Act, 57-8a is the Community Association Act, 16-6a is the Nonprofit Act)
HOA resources and laws annotated
HOA resources and laws annotated