Mitochondrial Eve is a theory that all modern humans came from one woman over 140,000 years ago. As Mitochondrial DNA is directly passed only from mother to child and has no recombination (like that of your typical chromosomes) you can use the mutations and mutation rates to trace back when and where your ancestors diverged. As of now the information suggests that all Modern humans came from this "Eve" who lived somewhere in Africa. As men do not pass on any form of mitochondria to the child they can be listed as Mitochondrial dead ends. A similar process is used to determine the "Adam" of all men using the Y-Chromosome.