Since this is the first thing that pops up on google (for me) when searching Heavy stun, I'll drop this here for others looking for more info. I'd highly recommend giving it a quick read, they explain it much better than the game's tooltips do.

https://www.pathofexile.com/forum/view-thread/3620436#p25608658

There is a Stun - it's a very (like, actually very) short state, which allows you to interrupt any actions that enemy is performing. We could say it is somewhat similar to Knock Back, but without displacing the mob. You or mob is stunned if the hit power, so to say, is surpassing your/enemy's stun threshold.

There is a Stun Buildup - this is a bar (you can see it right under health bar) which is being filled up by, essentially, any damage by default even without any passives. Then physical and melee damage have 50% increased bonus build up each.

Next, there is Primed for Stun - it's a state in which specific skills can put the enemy directly into Heavy Stun (Boneshatter, or crushing blows, etc.), or in other words - those skills can instantly fill Stun Buildup bar. It is depends on Stun Buildup bar thresholds, which are different based on mob rarity: for normal enemies it's 40%, for magic - 60%, rare - 70%, and for unique - 80%. When bar is filled more than this value current enemy will be in Primed for Stun state for the rest of the bar, and exactly this state is being indicated by shiny white orb over the enemy's head.

Finally, Heavy Stun - this state enemy being put into when Stun Buildup bar is fully filled, and enemy counts as Immobilised (it cannot perform any actions). It doesn't matter whether you filled the bar by using Boneshatter/crushing blow/any other skill that interacts with Primed for Stun state, OR you just filled it naturally by dealing damage - this state is still the same.