What a "Pre-Lien" is depends upon the state. It can be a Preliminary Lien Notice that notifies the owner(s), lender(s) and primary contractor that you have been contracted to provide material, labor or services for the improvement of real property and will ultimately look to local lien laws to collect your money, should it be required.
Some states only require a Warning Notice be delivered prior to the lien being recorded. These notices warn that you have not been paid for work and will be liening the property within a specific time period.