Tarnish is a chemical reaction that causes a discolouration to the outer layers of certain metals including but not limited to copper, brass, silver and aluminum. It forms a bronze, grey or black film which can be easily removed by rubbing the metal with a soft cloth. A silver polishing cloth can be used to remove tarnish from sterling silver. Solutions are also available which can help to remove tarnish but they should be used with care because they contain harsh chemicals.
The film which covers the metal does not cause any damage and instead forms a seal so that deeper layers are protected from tarnishing. As the purity of silver decreases the probability of tarnish increases. Silver itself is not particularly prone to tarnishing at normal temperatures however other metals that are used in the alloy such as copper are.