CPU stands for central processing unit.
CPU's are the brains of a computer. Without CPU's computers wouldn't be able to do anything whatsoever. They were first invented in the 1960s, in the second generation computers but not accessible to everyone at first because they were expensive to make. As engineers and scientists continued there research they found cheaper ways to make CPU's so more people could use them and eventually made them smaller so by the end of the 1900s computers were now much smaller. As the CPU is one of the most important things it is also one of the things that makes computers so expensive.