Metal is a resource, one of the 3 main resources in the game. It is used in almost everything, especially buildings.
Metal however can often be in excess of other resources, due to the fact that it is extremely easy to obtain and that Metal Mine outputs may greatly exceed the output of a Crystal Mine at a comparable level. Metal is fundamentally the most important resource in the game, as you cannot do much in OGame without Metal. However metal loses its usefulness later in the game, where a planet with millions of excess Metal is not uncommon. This loss of usefulness can result in many players neglecting their Metal Mine, however resources should be equally managed. Despite this, keeping the Metal Mine a bit lower than the other mine levels can ensure that enough resources are spent where they are needed.
Metal can be produced in mines, captured in a raid, and harvested from a debris field. It is stored on your planet and can be spent on items such as ships, defense, technology and buildings. It is generally considered that Metal should be the first priority for a new player as it is so widely used.
The amount of metal produced by a metal mine per hour can be calculated with this formula: