Slavery existed in the United States many years ago. If you would like to know more about the history of slavery in the US, Google some of these:
The Underground Railroad
Harriet Tubman
Slavery in the South
There were many times and different kinds of slavery but the time of African slaves in southern United States seems to be the most popular. What we call "native Americans" or "American Indians" were also enslaved at an even earlier time when the country was first "discovered." However, the Indians were here first (not to be confused with Indians from India).
Good luck!