How is ‘decimated’ data different from ‘undecimated’ data?
"Decimated" is another word for "downsampling," or in other words resampling the original ("undecimated") data with a lower sampling rate. So the decimated data will have a lower sampling rate than the undecimated data.
Edit As pointed out in the comments, it is a very good idea to low pass filter the data before downsampling, to avoid aliasing effects. There seem to be different conventions as to whether low pass filtering is included in the definition of "decimation".