[ros-users] roswtf issue

When doing the ROS tutorial at URL: http://www.ros.org/wiki/ROS/Tutorials/Getting%20started%20with%20roswtf,

There is a message below that bothersome (highlighted in red).
Is it ok to ignore this?
If not, what should I do about it?
What does the tf package do?


sam@sam-laptop:~/ros/ros$ roswtf
Unable to load plugin [tf.tfwtf] from package [tf]
Stack: ros
================================================================================
Static checks summary:

No errors or warnings
================================================================================
Beginning tests of your ROS graph. These may take awhile...
analyzing graph...
... done analyzing graph
running graph rules...
... done running graph rules

Online checks summary:

No errors or warnings