A preliminary notice is a document a subcontractor or supplier sends early in a construction project to tell the owner, and often the general contractor and lender, that it is working on the property and could claim a lien if unpaid. In many states it is a condition of keeping mechanics lien rights, and it has a deadline counted from when you start work.
Florida calls it a notice to owner. Other states use other names, such as notice to contractor or notice of furnishing. This page is general information about US practice, not legal advice. Lien law is set by each state.