Sim Phone Whats The Point Of A Dual Sim Phone? What Happens If You Only Put One Sim Card In A Dual Sim Phone?

Whats the point of a dual sim phone? what happens if you only put one sim card in a dual sim phone? - sim phone

I want a phone as a touchscreen phone called the iPhone. which almost exactly like the iPhone, except it works on tmobile. But it is a dual SIM ... What does this mean? What is the difference between a dual-SIM card and a normal? and what happens if I put a SIM card into a dual-SIM?