This is a section of DNA which is inherited from the mother exclusively. Measuring the difference between the MTDNA of two specimens allows one to project how far one must go to find the common female ancestor of the specimens. The idea is based on a constant frequency of random mutations introducing differences to the DNA, accumulating generation by generation.
This method has been used for extrapolating the age of the common female ancestor of present day humanity. There is a similar section of DNA which is only inherited from the male line, thus the ages of both 'Adam' and 'Eve' may be estimated by this method. The estimates of age vary however quite substantially, thus Adam and Eve would have lived countless generations apart.
See DNA Changes