Anyone who improves a property and is not paid may be able to file a mechanics lien against it. A lien waiver is a signed document giving up some or all of those lien rights, usually in exchange for payment. Owners, lenders and GCs collect waivers so that money paid down the chain cannot come back as a lien.
Waivers come in four common types. Conditional waivers take effect only when the stated payment actually clears. Unconditional waivers take effect when signed, whether or not the money arrived. Each can be progress (covering payment through a date) or final (covering the whole job). A typical rhythm: a sub sends a conditional progress waiver with this month's pay app, and an unconditional progress waiver for last month's payment once it has cleared.
Several states require specific statutory waiver forms, and some require notarization, so the form that works on one job may not work on the next.
The practical problem is volume. Every sub and supplier, every month, every job. Missing waivers are one of the most common reasons a pay app stalls. A simple tracker that ties each waiver to a vendor, pay app and amount, and holds payment until it is received, prevents most of that pain.