Metal is the basic fuel that drives your industry. You will need lots of metal to build the armada of ships that you will need to conquer the galaxy.
Metal is produced by a world's mines. Since usually the metal is produced on worlds that don't have a lot of industry, you must load metal from outlying worlds onto your fleets, and then carry it back to your homeworld, so you can make use of it to build more ships.
Usually, there will be enough metal produced collectively by the worlds within easy reach of your homeworld to keep your industry working at its full capacity of 30 units per turn--but you will have to organize your fleets enough to be able to dedicate enough ships to the task of picking up all this metal and bringing it back to your homeworld each turn.
If you are lucky enough to meet a Trader early in the game, you may find them willing to do the work of bringing your own metal back to your homeworld. The Trader has an advantage in this, since they can do the same job with half the number of ships that anyone else can; and this is also the primary way a Trader earns points. For this reason, a player who enlists the help of a Trader may be able to build up their ships--and thus, their military power--more quickly than other players who have to haul their own metal. (But keep an eye on your Trader ally's score, so you can decide on a propitious time to end the alliance!)