From Diablo Wiki
Root refers to properties found on items, skills, and Elite Affixes that "root" or stick a player or monster in place. The most obvious types are Elite Affixes such as Jailer or Frozen, but stunning, blinding, or slowing effects can effectively function as rooting effects as well.
Root is a form of crowd control and being rooted is very dangerous in a game such as Diablo 3, where mobility and escapability are essential for survival.
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