please describe any achievements you have made since your last application."
If you applied and did not get in, it is because of some lack of something or other. This essay should show that you have taken action to improve your preparedness. Show them that you are making a strong effort to improve whatever shortcomings you had in the past.
Show that you are particularly interested in a specific specialization, and show that you have been doing a lot of reading. That's my best advice! :-)
Don't just list achievements, but instead tell them about your view of this situation in which this program is a crucial stepping stone, and show that you know what your areas for improvement were and that you took proactive steps.