What is Dual SIM?
Dual SIM feature on a device means the device can hold 2 different SIM cards at the same time. Dual SIM smartphone can use either of the two SIMS to initiate or to receive phone calls. Dual SIMs are useful for separating personal and business calls in locations where lower prices apply to calls between clients of the same provider.
You can leave the other slot empty and only fill it with SIM card when you need to. For example, when you go to travel for vacation or business purpose, you can get the local country card and use it without taking out your own SIM card.