A compiler is a program that converts code written in one programming language into another. It might take code written in Ruby and turn it into C, or code written in JavaScript and turn it into Machine Code. The code accepts as input is known as the source language. The code that it outputs is known as the target language. Most programming languages are written for people to understand and be able to write. Unfortunately, computers don't understand humans very well, so languages that are easy for us to read or write are impossible for computers to read and understand.
