It is the distance that an athan would reach in a place where no concrete buildings or modern cars and noise making machines are there.

In a recent question you said someone must attend the congregational prayers in the mosque if they lived 3 – 5 km away from the mosque. What is the basis of this distance ? Does this assume transport by car or only on foot ? If it relates to the distance the adhan can be heard (possibly with amplification) the adhan can’t be heard 3 km away !!!