Dual citizenship is basically when an individual is a citizen of two countries simultaneously. Some California residents might hold dual citizenship if they were born in the U.S. to immigrant parents or if they were born in another country to a citizen of that country and a U.S. citizen. Someone who has become a naturalized U.S. citizen can also seek to regain citizenship in his or her country of origin, which will cause the individual to have dual citizenship.
Someone with dual citizenship does have several advantages over someone who is a citizen of a single country, such as the ability to become a resident of either country without excessive legal complications. However, there can be drawbacks as well; someone with dual citizenship might have to pay taxes to two different countries, regardless of where the individual lives. Each country might also have residency requirements.