Tarnishing is the slight corrosion of the surface and is evident as a dark discolouration of the item, also called a tarnish film.
Gold is one of the least reactive chemical elements. Gold alone or pure gold does not combine easily with oxygen so it stays shiny, it does not rust nor tarnish, again, that is pure gold. In jewellery, it is rare to find a piece that contains only the pure gold element. Pure gold or 24 karat gold is too soft to be used in jewellery so it is usually alloyed with other base metals altering its properties. It is these other base metals alloyed with gold that actually reacts with or to oxygen, sulfur and moisture that eventually tarnish your jewellery.
When your jewellery tarnishes, it doesn’t mean that it’s not real; it is just not pure gold or 24 karat gold. Real gold jewellery can also eventually tarnish. There are even rare circumstances in which jewellery containing high karats of gold have tarnished but by and large the higher the percentage of gold in the alloy, the less likely that the gold piece will tarnish. When lower karat gold jewellery tarnishes, it does so in a much slower phase than sterling silver.