People are sometimes confused by the concept of wrongful termination, particularly because termination is not always fair. In California, you can be fired for any reason and at any time by your employer as long as that reason is not illegal. Of course being fired for no good reason is extremely unfair, and you may question the legality of it, but there are very clear instances in which a firing is illegal and thus wrongful.
Any kind of termination that is related to discrimination is generally unlawful. If your employer had a history of mistreating you or discriminating against you for race, gender, religion or any other such issue, then you could claim that they fired you because of this discrimination. Obviously if you can prove that they fired you specifically because of some kind of discrimination, you will have a very strong wrongful termination case on your hands.
There are other illegal reasons that an employer may choose to fire you for, including taking advantage of your rights. Employees are protected from retaliation firing, meaning that if you discover your employer doing something illegal and you report this illegal activity to the authorities, it would be illegal for your employer to fire you for this. It can also be illegal if an employer fires you for complaining about unsafe working conditions or being involved in a union.
Obviously, it is not always easy to prove that your employer fired you for an unlawful reason, which is why it may be in your best interests to hire an attorney. An attorney who is familiar with employment law can help you understand your rights and offer insight into your case that may help you prove that you were wrongfully terminated. This could lead to compensation and even allow you to get your job back.