Drunk in Public is defined by California Penal Code Section 647(f). In general the definition is that a person is drunk in public if he is too intoxicated to care for his own safety or the safety of others. This is a very high standard.
Many times the police use this as a method of simply clearing the streets. The reality is that the charge very well might not stick. In other words, the police can take you into custody and charge you with drunk in public but one of two things may happen: the prosecutor may reject the charge or the charge may not be proven in court.
If you are charged with this, it is essential that you consult with an experienced criminal law specialist immediately. It is quite possible that your attorney can get the charge dismissed. If that isn’t possible, there are often alternatives such as paying court costs and/or attending drug and alcohol classes that can be employed in order to earn a dismissal.