Dual citizenship is the ability to legally carry or procure legal documents, live, work, etc.. in two separate countries. It means that you can live and work in both countries without obtaining visas. You can also hold valid passports, drivers liscense etc.. of both countries. Also, you become subject to the laws of each country while in them, meaning you wont be extradited to your home country. Some countries may allow you to keep your originalcitizenship, and obtain citizenship in another country. This is especially true if you have relatives from that country. They can ask you to relinquish your previous citizenship to gain your new one. So what your mom means is dont give up your old one if you move to another country for a while, because you will have to reapply in your original home country.