Short Stun Mechanics Guide /Learn how Stun Threshold works

Taking a hit has a chance to stun you based on your total stun threshold (up to 100% chance to get stunned if a hit's damage is equal to or higher than your threshold), stun threshold is based on your total life pool by default, but you can take nodes on your passive tree or add jewels with mods that can increase your stun threshold in different ways.

For example 30% Additional Stun Threshold based on Energy Shield adds 30% of your ES pool to your stun threshold pool, this is a great node for Chaos Inoculation builds, Sapphire jewels also have this mod up to 15% of your ES.

There are also nodes that increase your stun threshold multiplicatively, but if your life pool is low or you allocated Chaos Inoculation, you need to take other stun threshold nodes like the ES based ones, so you have a decent stun threshold amount to begin with to take advantage of generic stun threshold increases.

If a hit deals damage to your ES, you have a 50% chance to avoid it by default.

You have a 50% increase to your stun threshold each time you were stunned in the past 4 seconds

If a hit deals damage equal to or higher than your total stun threshold, it is guaranteed to stun if it passes the 50% avoid for ES check.

And that's the stun threshold crash course -SleepyTiel