What is Compiler?
A compiler is a tool which has the ability to read the source code and translate it to object level code.
Pictorial Explanation
Why do we need compiler to execute the program?
Because computer can't understand the source code directly. It will understand only object level code.
Source codes are human readable format but the system cannot understand it.
So, the compiler is intermediate between human readable format and machine-readable format.
The compiler will parse the source file and translate it into machine understandable object file.