What Yaroslav is referring to is known as a software design document. For any project more complicated than"hello world" you should have one. In fact the software design document for "hello world" is "Write a program that prints 'hello world' to the screen.". If you don't have one how do you know when the job is done or it meets the requirements.
See
https://en.m.wikipedia.org/wiki/Software_requirements_specification
https://en.m.wikipedia.org/wiki/Software_design_description
