An "era" is a period of history in which something important happens. An "era" usually lasts for a long time: hundreds or thousands of years. Some examples of well-known eras are:

The Victorian era (in Britain in the 1800's)

The Cold War era (from the 1950's to the 1980's)

the Golden era of ___

the Modern era

When an important event happens, you can say that the event "marks the end of an era" or "marks the beginning of a new era":

Americans hoped that the election would mark the beginning of a new era for the country.

The word "marks" means "symbolizes". So the important event is like a sign that an era is ending or beginning.