What does it mean if a condo association puts a lien on your condo if you haven't been able to pay your asessment fees?

As an owner, it means that your title to your unit is 'clouded' -- your title is encumbered/ not clear -- by the amount of your assessments past due and accumulating, that remain unpaid.
A lien may also appear on your personal credit report if the title is in your name.