What is a double insulated power tool?

inside a power tool is a roll of copper that is covered in a very thin layer of plastic to stop the machine from breaking down if too much dust builds up on it,double insulated means that there is two layers of plastic on the copper to increase the life of the tool

Class II or double insulated tool is one which has been designed in such a way that it does not require a safety connection to ground.

It is for safety not dust