Basically the ability to take a hit and not get stunned, the way Dark Souls does it is as follows:
Poise is a sort of invisible stamina bar, the more poise you have, the bigger the "bar"
Every time you get hit, the bar goes down, different weapons do different poise damage
A small broken sword? A very small amount of poise damage
A giant Zweihander? A **** ton of poise damage
Say you have 59 Poise, and the Zwei does 70 Poise damage, if you get hit, YOU WILL GET STUNNED
The bar does fill back up though, it takes 3 seconds before it starts "filling" back up, and it takes roughly 4 seconds to "fill" back up to max
25% per second
40 Is a decent amount, and the Wolf ring grants you 40 at the cost of a ring slot
(very worth it though, 40 poise for no extra weight? Nice)
played dark souls for the first time couple days ago, only had to look up where the bonfire was at the top of the fortress because I had to go. Honestly, it wasn't as difficult as I'd thought, from people's impressions. Besides Ornstein and smough, which took me longer than it should have