That means the MOST RECENT individual that lived that stands in the direct line through mothers with the set of individuals in question. If we are looking at the entire population of people today, the MRCA of that set is called mitochondrial Eve. The matrilineal MRCA of humans is estimated to have lived between 150,000 to 200,000 years ago. (note that there are other MRCAs; eg the MRCA as measured by the X-chromosome - since the X-chromosome descends through fathers and mothers, the lineage is different - but there is still the concept of the individual from whom the X-chromosomes of all people alive today descended. That is not the same as matrilineal MRCA or Mitochondrial Eve).