OK, Dielectric grease is an insulator by definition, before you comment look it up for yourself.
I know what some of you are saying, "Well wise *** if it's an insulator how can it work for electrical connections?"
Here is the answer, it is also a low pressure grease. that means that when squeezed it pushes out of the way and does not leave a film. So when you put together, say a bulb and a socket, the bulb pushed away the grease making a good contact with the socket metal and the residue left around the connection will seal the metal from moisture.
When used with rubber covers like modern spark plug covers, only the rubber needs to be greased, not the metal as the moisture will be kept away from the actual metal by the rubber and grease.