Legal Question in Real Estate Law in California

It means an unlawful detainer action (Eviction Lawsuit) has been filed and the defendant/tenant will soon be served with a Summons/Complaint, if he/she hasn't been served already.

Not sure exactly what you mean. Most unlawful detainer actions begin (are filed) when a tenant fails to vacate after receiving a notice such as a three-day notice to pay rent or quit; then the next "notice" is the service of a summons and complaint. See Code of Civil Procedure section 1161 for the key provisions of law regarding initiating a UD action.

Mr. Shokrian is right. It is a notice that is mailed to a tenant by a court after an unlawful detainer action has been filed. An unlawful detainer action is a court proceeding filed by a land owner to evict a tenant.