What happens when source file is assembled?

In reference to programming, a source file, which contains high level code, such as c++, visual basic, or any of a myrid of laguages, is assembled by a compiler, it is turned into a language known as assembly, a low level machine language which is barely human readable (And many times longer than the original code), this form of code is read by the machine to exacute the program. The point of high level code which is found in the source files is to make programming simpler, therefore allowing for more complex applications.