DR (or damage reduction) 5/bludgeoning means that anything other than 'bludgeoning' damage will do 5 less damage. So any weapon listed as bludgeoning (clubs, maces, etc.) will bypass the damage reduction.
The type of damage after the / is what type of damage bypasses that DR. The number is how much is shaved off of other attacks.
So, a skeleton that has DR 5/Bludgeoning will take 5 less damage from Piercing or Slashing damage, but takes full damage from Bludgeoning weapons.
Additional notes:
Magic is not usually subject to DR, as long as it is energy damage. For example, fire still goes through the DR 5/Bludgeoning. However, Magic that replicates physical damage types, such as piercing, slashing or bludgeoning damage is subject to it, just as actual physical attacks would be.
DR with the /— type means that it is not overcome by any type of physical damage, though energy damage still goes through as normal.
Also, ninja'd!
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Bludgeoning weapons do full damage. Weapons that are solely slashing and/or piercing do 5 points less damage on each hit.
Energy damage (fire, etc.) and most spells ignore DR, though the creature might also be resistant or immune to some energy types. Skeletons, for example, are immune to cold. As undead, a number of spells don't affect them.