Dual sim means two sims. Dual sim is the use of two SIM on a phone. Dual sim system such a system that supports the use of two SIM at the time in a phone. The phone with which we can use two SIM is called dual dim phone. It has two sim slots to setup to sim. Hardware and network system provides for using dual sim on the device.
When the second sim setups on mobile, it allows to user to use manually two mobile network systems. Usually, users use two SIMs for two types of tasks, like one for personal and another for other purposes.