Autocracy is a form of government. In an autocracy, a single person has all legal and political power, and makes all decisions by himself or herself. The person who holds the power is called an autocrat.
When there is a monarch ruling a country as an absolute monarchy, this is also called an autocracy. Having an autocratic government does not mean the country is governed well or governed poorly. It is just a description on how it is set up.
In modern times, most autocrats gain power as part of a larger nationalist, communist, or fascist movement. Once in power, they eliminate all other kinds of authority in the country, such as judges, the legislature, and political parties.
Historical examples of autocracies include: The Aztec Empire in Mesoamerica, in it, the Aztec Emperor functioned as both the sole ruler of the Empire himself, as well as of its military, he also was the the religious figurehead behind the empire's aggressive foreign policy (In this time, the priesthood supported a pantheon that demanded human sacrifice, and the nobility consisted mainly of warriors who had captured many prisoners for these sacrificial rites), Eastern Han under Dong Zhuo, the Empire of Japan under Hirohito, and the Roman Empire, in 27 B.C., Augustus founded the Roman Empire following the end of the Roman Republic. Augustus officially kept the Roman Senate while effectively consolidating all of the real power and influence in himself.