A notice the contractor or supplier sends early in a job, preserving the right to later file a mechanic's lien if they aren't paid.
Almost every state with strong lien laws requires you to send a preliminary notice — sometimes called a Notice to Owner (NTO), a 20-day notice (California), or a Preliminary 20-Day Lien Notice — within a short window of starting work, even if you've been paid.
It doesn't mean you're going to file a lien. It just preserves the option. Skipping it (or sending it late) usually means you lose lien rights on that job entirely, even if you never get paid.
Subs and suppliers benefit the most — they often don't have a contract with the property owner, only with the GC. The preliminary notice is what puts the owner on notice that the sub or supplier exists and is entitled to be paid.