The Preliminary or 20 Day Preliminary Notice is a form that is served or given within 20 days of a claimant, other than the Direct Contractor, providing labor or materials to the private works Project. Civil Code Section 8204 provides:
"(a) A preliminary notice shall be given not later than 20 days after the claimant has first furnished work on the work of improvement. If work has been provided by a claimant who did not give a preliminary notice, that claimant shall not be precluded from giving a preliminary notice at any time thereafter. The claimant shall, however, be entitled to record a lien, give a stop payment notice, and assert a claim against a payment bond only for work performed within 20 days prior to the service of the preliminary notice, and at any time thereafter.
(b) A design professional who has furnished services for the design of the work of improvement and who gives a preliminary notice not later than 20 days after the work of improvement has commenced shall be deemed to have complied with Section 8200 with respect to the design services furnished, or to be furnished."
https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=8204.&lawCode=CIV
It is notice to the Owner that the subcontractor or material supplier is providing work or material on the Project and if not paid can and will place a mechanic's lien on the property or serve a stop notice to assure payment of what is owed if not paid by the Direct Contractor.
While it should be served prior to providing any labor or materials to a Project it can be served at any time and all labor or materials from 20 days prior to the service up to completion of the work or delivery of materials is covered by the Preliminary Notice and if the claimant is not paid for that work or material they can file a mechanic's lien or serve a stop notice for the amount provided from 20 days prior to the end of contributions.
If a Preliminary Notice is served on the Owner the Owner would be wise to obtain Conditional and Unconditional Releases from that subcontractor or material supplier when payments are made to the Direct Contractor. Doing so will assure that the Owner does not end up with a mechanic's lien on its property or a stop notice served at the end of the Project. The CSLB provides information and forms for the Conditional and Unconditional Releases: