What do they mean when they say you should use any descriptive method to describe the data?I have data from the 2001 census and I have to write a...

What the assignment is suggesting is that you incorporate
the data into the written exposition of the essay/report that you are writing.  You
should state whatever piece of data you are referencing, but you should use complete
descriptive sentences to not only report but to
connect that data to whatever argument you are proving. 
You should be describing the significance of the data, not
merely stating the data.  Presenting data in this way creates a more seamless connection
to the analysis or argument of the essay.  If you were to present the data in a chart,
for example, then the reader of the report has to interpret the data before going on to
read whatever you might want to say about it.  By incorporating the data into your
essay, you are forced to select only the most relevant piece of information and
immediately explain it, but it keeps the focus of your essay on
target.