Original: The term "prehistory" can be used to refer to all time since the beginning of the universe, although the term is more often used to describe periods when there was life on Earth and even more commonly, to the time when human-like beings appear on Earth.
Simplified: Pre-history can start at the start of the universe but the word more often means a time when there was life on Earth; dinosaurs are called prehistoric animals and cavemen are called prehistoric people.