Anno Mundi means "in the year of the world." It is a way of counting the years that dates all the way back to the beginning of time. The concept comes from Jewish history and religious beliefs. The story goes that when God created the world, he also created the first man, Adam. Every year since then has been counted as a year in the history of the world.
Anno Mundi is used to date important events in Jewish history, including the creation of Adam and Eve, the building of the first temple in Jerusalem, and the exodus of Jews from Egypt. It is also used in other historical records and calendars, but not as commonly as the standard Gregorian calendar used today.
In the Anno Mundi system, the year 1 is the year of creation. This is different from the Gregorian calendar, which counts the years since the birth of Jesus Christ. So, for example, in the Gregorian calendar, the year 2021 would be the year 2,021 since the birth of Jesus. But in Anno Mundi, the year 2021 would be the year 5,782 since the creation of the world.
In short, Anno Mundi is a way of counting the years that goes all the way back to the beginning of time and is used to date important events in Jewish history.