Seed media is basically anything from a cycled tank you place in an uncycled tank.
Cycling a tank is simply the process of establishing a colony of (what I'll call) ammonia bacteria (they convert ammonia to nitrite) and a colony of (what I'll call) nitrite bacteria (they convert nitrite to nitrate).
To begin the process of cycling a tank, all you have to do is add ammonia to the water, start running your filter and sit back and wait. This ammonia bacteria and nitrite bacteria are in the air we breath. Eventually a single ammonia bacteria will find it's way into your tank, begin feeding on the ammonia, and start growing a whole colony of ammonia bacteria in your tank. That process will start putting nitrites in your tank. Eventually a single nitrite bacteria will find its way inside your tank, begin feeding on the nitrite and grow a nitrite bacteria colony.
But starting from a single bacterium will require weeks to grow these colonies.
To speed the process along, you take some 'seed media' (basically anything from a cycled tank like a hand-full of gravel, or a filter pad) that already has a colony of a million ammonia and nitrite bacteria on it. Now rather than taking weeks to grow an ammonia colony from a single bacterium, then wait weeks to grow a nitrate colony from a single bacterium, you jump start the process with a small colony from the 'seed media' and your tank cycles in days rather than weeks.