I did a quick cross-reading of the article you linked to
Well if an object is moving the frequency of the electromagnetic waves reflected from the moving object is changed a little bit.
The articel says:
Frequency of Doppler shift is proportional to velocity of motion. Typical human walking generates Doppler shift below 100 Hz.
This video seems to explain it pretty good
The number "16000400.0 " looks like related to the clockfrequency of the microcontroller which is 16 MHz
The electromagnetic frequency of the sensor is an ultrahigh frequency of 10500 MHz
about "lfrq" my guessing is: low-frequency = the lower frequency which is called the doppler-shift-frequency
pp seems to be the period of the doppler-shift-frequency
best regards Stefan