Here are some terms that Ralph Taylor provides to help you understand your reading of articles and blog posts about genetic genealogy.
MRCA: The most recent common ancestor shared by two or more persons with a DNA match. We are concerned only with the most recent, because preceding ancestors will also be shared.
TMRCA: The probable time to the MRCA, expressed in generations.
Probability level: The cumulative probability, expressed as a percentage, that a MRCA lived within the specified number of generations. Here, we give three probability levels, 90%, 70%, & 50%. 90% means the chances are 9/10 the TMRCA is <= the given number; 50% probability is as likely to give a low TMRCA as a high one.
Genealogic time frame: A time for which it is feasible to specifically identify an ancestor, generally from about 1350 AD to the present.
Generations, length of: This is an estimated average of 25 years, developed from 18th and 19th century America statistics. We use the estimate to convert generations to common dates. Other societies may have had a different generational length -- either longer or shorter.
<=: Means less than or equal to, the maximum generations number for that probability level
>=: As applied to dates, means no earlier than the given date.