What does it mean when iPhone 5 shows SIM Not Supported

What does it mean when iPhone 5 shows SIM Not Supported

That it is just not supported?

SIM Not Supported means your iPhone is locked to a certain carrier which means you can't use SIM Cards from another network. You either need to use a Sim Card from the network it is tied to, or you need to get it unlocked. Depending on which network it is tied to, it may cost money to unlock the iPhone, some networks offering unlocks for free.