Najun Geolum(Low Walking)
Refer to this explanation page for more detailed understanding.
This training is very important for every technique. (See Ch.3-2 of 3JKY for your understanding why.)
If you read every chapters of 3JKY(Taekwondo Bible, Vol.2) and reflect its meaning, you will get more exact technical points of this motion and training.
You should be able to walk this way for 100 meters at once.