Torben Ægidius Mogensen's "Basics of Compiler Design, Chapter 1: Introduction"
Read sections 1.1, 1.2, and 1.3. Note that "compile" means to translate from a high-level language, used by humans, to a low-level language used by a computer. A high-level language uses concepts, objects, and tasks performed by a human, whereas a low-level or machine language uses concepts, objects, and tasks performed by machines.
The input language to a compiler, typically a programming language, is called the source language. The output language of a compiler is called the target language or object code, typically an assembly language or machine language.
The author describes seven phases of a compiler. The middle phase is called Intermediate Code Generation. The intermediate code is independent of a particular target machine. One of the back-end phases is called Machine Code Generation, which translates the machine-independent code to machine-dependent code for a particular computer.
The need for compilers arises from the need for high-level languages, which are more relevant for problem solving and software development by humans. Moreover, high-level languages are machine independent.