Q:

What does hardware virtualization extension mean? 

When programming againt virual machine, some system remind hardware virtualization extension is needed.  What does it mean? 

A:

Hardware virtualization extension allows your computer to have second state which represents virtual machine state (for example vmware).  When VM code is scheduled to run, processor switches to its "virtual" context and then works in this "sandbox".  When hypervisor executes guest code it needs to emulate many hardware aspects - perform software virtualiztion of them.  Hardware extensions allows to do the emulation in hardware.  It significantly reduces virtualization overhead.