It usually refers to the time before 1492, which is when Columbus arrived in the West Indies.
Native people in North and South America before Coloumbus arrived there.
Pre-columbian means before Christopher Columbus arrived. Explorations came after.
They were basically American societies before Columbus. Examples being the Aztec empire, the Inca empire, Various Mayan city-states (Tikal, Palenque, Chitzen Itza etc) and others (Tlaxcallan, Moche,...
They lived in parts of Mexico, near the Yucatan Peninsula. They also lived in parts of Central America.