there are 27 nakshatras or constellations which are the 27 equal divisions of the lunar path. The path treaded by the moon in the year is equally divided into 27 divisions and each sector comprises of a Nakshatra which is further divided into four charanasor the four steps.
Out of 27, the ones ruled by the planets Mercury and Ketu are the ones which are termed as Mool Nakshatras
AS a remedy mitti / soil from 27 villages is used along with some other items to remove the malfic effects of mool born