I'm sunning Sunless Citadel and coming up to the climax/boss battle.  One of the actions the villain NPC can make as written in the adventure "Sir Braford uses Shatterspike  to destroy his foe’s weapon, if possible."

Shatterspike:

You have a +1 bonus to attack and damage rolls you make with this magic weapon. If it hits an object, the hit is automatically a critical hit, and it can deal bludgeoning or slashing damage to the object (your choice). Further, damage from nonmagical sources can’t harm the weapon.

 Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

 I know about material AC/HP but nothing how making that type of 'called shot' to a target object so what I'm trying to understand is how a player/NPC can target a foe's weapon because I thought there was not a way to target an item worn or carried.

Any clarifications on how to effectively use this weapon would be appreciated. 

As far as I know he would have to disarm them first. Didn't see anything in statblock about that, maybe environment?

You might want to give this thread a look.
https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/81128-headache-ruling

Quote from Emmber >>

You might want to give this thread a look.
https://www.dndbeyond.com/forums/dungeons-dragons-discussion/rules-game-mechanics/81128-headache-ruling

 Oh, that was very useful and current!  A lot of my personal misgivings about the weapon were mentioned in that thread so it is nice to know I was not alone.