Most California workers - from hourly to executive - are at-will employees. Do you have a full understanding of what it means to be at-will?
If you check your employee handbook or the documents you signed on your first day of employment, you will probably find that you are an at-will employee. Exceptions to at-will employment are usually outlined in employee contracts for a specific job duration or sometimes in union contracts.
If an employer makes statements like "you have a job here for life" or "we only terminate people for just cause," the employer has possibly made an exception to at-will employment. The employer may have made an "implied contract" which, if proven, is just as enforceable as a written contract.
If you are an at-will employee - you may resign from your employment at anytime for any reason with or without notice. Likewise, your employer may terminate you with or without notice for any reason - except for an illegal reason.
An illegal reason would include discrimination for age, race, gender, disability, etc., or some other type of wrongful termination including retaliation for whistleblowing.
Also, it is illegal and against public policy for an employer to terminate you for exercising a legal right such as going to jury duty, taking family leave or filing a workers' compensation claim.
At-will employment, on the surface, looks like a fairly straightforward concept; but like most employment and labor laws there are many complications and exceptions.