What kind of substance is a Charge.They say its invariant?

Answer 1

Charge is a property of atomic particles, the buidling blocks of matter. It can be positive (proton), negative (electrons) or neutral (neutron). Molecules can also be charged. Charges cannot be created or destroyed, they can only be added or subtracted (by adding or subtracting electrons usually). Its like the color of your hair. It could be brunnet, red or black. You can add pigments or it can fade several days later. But you cannot change a person's hair color permanently (of course some turn white when they get old though). So in this sense it is invariant.

Answer 2

Your hair color is not invariant. What invariance usually means is with respect to a boost to a relativistic inertial frame. So red hair in one frame may appear blue in another. However, charge is invariant, so 5 coulombs in one frame looks like 5 coulombs in another frame. Charge is just an intrinsic property of things, and most like comes from quarks for protons and neutrons, and God knows what for electrons.