what is the meaning of transformGlobalPlan in goalfunction.cpp line 80

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/**

• @brief Transforms the global plan of the robot from the planner frame to the frame of the costmap,

• selects only the (first) part of the plan that is within the costmap area.

• @param tf A reference to a transform listener
• @param global_plan The plan to be transformed
• @param robot_pose The pose of the robot in the global frame (same as costmap)
• @param costmap A reference to the costmap being used so the window size for transforming can be computed
• @param global_frame The frame to transform the plan to
• @param transformed_plan Populated with the transformed plan */

bool transformGlobalPlan(const tf::TransformListener& tf,

const std::vector<geometry_msgs::PoseStamped>& global_plan,
  const tf::Stamped<tf::Pose>& global_robot_pose,
  const costmap_2d::Costmap2D& costmap,
  const std::string& global_frame,
  std::vector<geometry_msgs::PoseStamped>& transformed_plan);

what is the meaning of this function? I have got the global_plan from the globalPlanner in the global_frame,have I? Why and when should I call this function? and what is "the planner frame",Does it equal to the global_frame?