You can play a Dark Souls game without giving a single thought to the poise stat, but if you're interested in post-game PvP shenanigans then you probably obsess over it. Basically, it determines how resilient you are to being staggered, and it's especially useful for tank builds. If you've got high poise, your attacks are less likely to be interrupted by your opponent's.
The poise stat does exist in Dark Souls 3, except it doesn't seem to do much. Since the game's release last month, users have been complaining that From Software appears to have forgotten to switch it on, but according to a statement by Bandai Namco sent to Kotaku, the poise stat barely works by design.
“The poise stat is working as intended and is not ‘turned off’ as some fans have theorised,” the spokesperson said. “The stat works differently than in past games and is more situational, which seems to be the reason for the confusion.”
So it looks like those hoping to wield high-poise builds in the same way they did in Dark Souls 2 are out of luck. If you don't know what poise is, or just don't know what you're doing at all, here's our Dark Souls 3 PvP guide for your convenience.