Some clothing has stun resistance now, it gives a small percentage chance that a stun spell won't work. Say, 3% or 5% stun resist. Also, there are a couple of different stun resist talents that pets can have (one is 7%, one is 10%). When you put them all together they can add up to 20% or more.
With a combination of gear and pets, it might be possible to have 25% stun resistance, which gives any stun spell a 1 in 4 chance of failing.
Stun resistance is like any other stat like accuracy, damage, or normal resist. You can get them on certain types of gear and the spell "conviction" (the trained spell version gives 90% stun resist as well as 20% critical block I think, and the treasure card gives 95% stun resist).
You can also get a square jewel with the stat and affix it onto an amulet, athame, or ring. My death wizard has a 3% stun resist on her morganthe amulet. It's also a pet talent, so if your pet has it in their pool, and if you're lucky you could end up getting stun resist a little bit of stun resist.