Poise is a game mechanic in Dark Souls.
Poise affects how difficult it is for a character to be staggered. The higher the poise someone has, the more difficult it is to stagger them. Large, heavy armor sets such as Havel's Set are known to grant the highest poise, while light cloth sets grant the lowest poise. The Wolf Ring grants additional poise to players and is the only ring to do so.
Players with high poise can still be staggered by weapons, but can take more hits before this happens.
Generally, large and heavy weapons can stagger the most easily. A claymore should have no trouble staggering someone wearing low-poise armor such as the Maiden Set. Conversely, smaller and speedier weapons such as the Bandit's Knife have trouble causing stagger.
Non-character and non-humanoid enemies have poise, as well. Generally, larger and tougher enemies boast higher poise than smaller enemies. Certain enemies simply cannot be staggered, although these are in the minority.
In PvE, poise is useful when facing strong enemies that hit hard and often, especially for tank classes that do not have the ability to dodge due to being weighed down by heavy equipment.
In PvP, poise is essential when fighting against large weapon builds, which are very common. Not utilizing high poise runs the risk of being caught in a stun-lock combo which could be anywhere from very damaging to lethal. Poise is less useful against small weapons users, but some poise is still important as being stunlocked by small weapons is possibly even more fatal.
High poise is not always a good thing. Wrath of the Gods, for example, is a miracle that causes heavy damage and stun. People with low poise will simply get blasted off their feet, and benefit from invincibility frames while they are getting up; on the other hand, people with high poise will get staggered, setting themselves up for another attack.