Elemental enchantments on weapons do precisely as they say: Add "X amount" of "Element" Damage. The elemental damage is in addition to your weapon's base damage; your weapon will still do physical damage equal to its base damage, but will also do extra elemental damage equal to the glyph enchantment. However, the extra elemental damage doesn't always "proc" or occur on every hit. There's an internal cooldown time of a few seconds after each time it procs. The "Powered" Trait is a researchable weapon modification that increases how often weapon enchantments will proc.

So, in short, If you enchanted your melee weapon with that Frost Glyph you found, your weapon would occasionally (once every several seconds or so) deal 30 cold damage to enemies in addition to the weapon's base damage. The glyph essentially gives you the opportunity to have your weapon do bonus damage.

I'd also like to note that you also have a chance to do an effect when the elemental attack procs. Such as ice freezing an enemy, flames setting the enemy on fire, lightning shocking them, etc. So it's not only a potential bonus damage on hit.

So, as a DK, if you have the "Kindling" passive from Ardent Flame, you do 33% or 66% bonus damage if your flaming weapon sets an enemy on fire (not every time it does fire damage). The key here is where it says "burning effects".