Armor Mastery (Ex): At 19th level, a fighter gains Damage Reduction 5/— whenever he is wearing armor or using a shield.
What overcomes this Damage Reduction? Nothing?
Nothing overcomes this Damage Reduction (besides things that completely ignore DR, like energy damage). That's what the dash means.
The numerical part of a creature's damage reduction (or DR) is the amount of damage the creature ignores from normal attacks. Usually, a certain type of weapon can overcome this reduction (see Overcoming DR). This information is separated from the damage reduction number by a slash. For example, DR 5/magic means that a creature takes 5 less points of damage from all weapons that are not magic. If a dash follows the slash, then the damage reduction is effective against any attack that does not ignore damage reduction.