It means that vacuum is not synonimous with nothing




It means mass
I used the "... " since matter is not a scientific term even if it is often used as synonimous with mass



It is right to use two separate terms for mass (matter) and energy but its wrong to suppose that they are two separate entities

Claiming that God ( energy ) sacrificed himself to create the universe ( matter )is not acceptable neither from a religious nor from a scientific point of view

Our intuition and classic physics before the Einstein's revolution assumed that energy and mass ( matter) were two separate items but they are not

This is a common misconception of the equation

Energy and mass are the two sides of the same coin, it depends which side you are looking at
They alwayes get along in the ratio : C^2