"Powershell 2 host programs that have been compiled with CLR 2"

Hi, i was reading the Windows PowerShell System Requirements and stumbled over the following sentence:

"..."due to a change in the runtime activation policy in Microsoft .NET Framework 4.0, Windows PowerShell host programs that werewritten for Windows PowerShell 2.0 and compiled with Common Language Runtime (CLR) 2.0 cannot run without modification in Windows PowerShell 3.0, which is compiled with CLR 4.0."

Can someone explain that to me? What are Powershell host programs?

Thanks, Rob