CDC stands for change data capture. This is used to 
implement incremental load approach in data warehouse. 
In this approach we traditionally keep a date field and 
pull data on incremental date values.
This assures that we are picking latest data (Or may be new 
batch data which ever is applicable).
Implementing this in mapping would require you to use 
parameter files which will keep HiWaterMark and LoWatermark 
which is repeatedly used to capture fresh data.