How to write better code? What is called clean code?

Answer 1

These articles may be helpful for "How to write good codes" :
► https://www.makeuseof.com/tag/10-tips-writing-cleaner-better-code/
► https://medium.com/@isaaclyman/steps-to-better-code-e6c3cce0c7f9
Now answering your second question, a clean code is code that is easy to read and to understand (applies to both the original author or somebody else). Well-written programs are easier to debug, easier to maintain, and have fewer errors.
This can be achieved if the person making the changes understands the code and the changes introduced in the code are not breaking any existing functionality.
If you think on a large scale then you are helping yourself by reducing the cost of maintenance of the project as the early the bugs are found, the less the cost of its maintenance. Cost, here in the sense, means both time and money. This is possible if you write neat and clean codes.

Answer 2

Clean code refers to code that is so efficiently written, that developers will have little problem reading, using and adding to your code.

There are a lot of books dedicated to the writing of clear and concise code, so you can read those if you want to know more (I suggest Robert Martins' "Clean Code")

Answer 3

Clean code complies the language's standards and is readable by an average experienced programmer of the language.