there is a code use odometry transform * tf::Vector(0,0, gravity) to correct imu gravity, how can i understand it ?

for example code:

void ImuOdom::updateOdom(const sensor_msgs::ImuConstPtr& data)
{

  // update time values
  last_time_ = current_time_;

  current_time_ = data->header.stamp;

  double dt = (current_time_ - last_time_).toSec();

  // update velocity in body-fixed frame
  tf::Vector3 lin_acc;

  tf::vector3MsgToTF(data->linear_acceleration, lin_acc);

  lin_acc -= odom_tf_*tf::Vector3(0,0,G_STD_); // correct gravity

...
}