What are STABLE ISOTOPES? HELP!!?

Isotopes are different forms of an element. It is when an atom gains/loses neutrons. Neutrons have a weight of 1 amu so the atom's weight increases/decreases but it's charge remains the same. The normal atom of carbon can be written like C-12 because it's m number is twelve. But an isotope of Carbon is C-13, meaning it gained a neutron, making it's m greater(1 amu greater). I hope that didn't cofuse you more :)