Hinder is a Debuff that slows Movement Speed in Path of Exile 2.

Hinder is a Debuff that slows Movement Speed in Path of Exile 2.

Hinder, or being Hindered, is a Debuff that reduces Movement Speed by 30% by default. Hinder can reduce Movement Speed by a different amount, but this fact will otherwise be stated on the Skill or modifier that inflicts it.
Modifiers that reduce Movement Speed stack additively with any increases to this stat. For example, if your only source of increased Movement Speed was '30% increased Movement Speed' on your equipped Boots, this modifier would essentially be cancelled out by an incoming Hindered Debuff, reducing you to moving at your base Movement Speed.
Note: Hinder can be inflicted on Monsters, and can also by inflicted on you by Monsters.
This is also the case for Monsters. If you inflict Hinder on Monsters, they'll typically have no additional increases to their Movement Speed and therefore they'd be moving at an effective 70% of their normal speed. However this is not always true, as some Monster Skills provide Buffs to their Movement Speed, commonly in the form of 'increased Movement Speed' which would stack additively with the 'reduced Movement Speed' from the Hindered Debuff.
There is also the scenario where you run a Map with a rolled Monster Haste modifier, which in part provides increased Movement Speed to Monsters. This increase is additive with the reduced Movement Speed imposed by Hinder, and therefore as an example, if Monsters you Hinder are Hasted for '27% increased Movement Speed', their Movement Speed would be 97% of the base speed after Hinder is taken into account.
Note: Hinder can be applied by any type of Damage, but is typically tied to Damage over time thematically.
The Hindered Debuff can only be inflicted if it's enabled by a modifier such as on the
Herald of Plague Skill.
Hinder lasts for 4 seconds by default, but a Hindered Debuff that's applied alongside a Damage over time effect will typically persist as long as the Damage over time effect is still dealing Damage.
The Hinder Support Gem can support Spell Skill to force any non-Ailment Damage over time that they deal to also inflict Hinder on Monsters.
The
Tornado Shot Bow Attack Skill creates areas of tornados that deal Damage over time to Monsters standing in them. Monsters that take Damage over time from these tornados will also be Hindered.