Some well-trained or naturally skilled warriors can traverse unhindered through the densest terrain.
Warriors with the Strider special rule are rare – normally, a model will have a subset of Strider, such as Forest Strider, River Strider or Marsh Strider and the rule only applies in terrain of the specified type. Warriors with the Strider rule do not have to taketests.
In older army books, this rule is presented as something along the lines of 'ignores movement penalties for [terrain type]'. Treat such rules as being the relevant version of Strider. For example, 'ignores movement penalties for forests' equates to the Forest Strider rule.