What does writing idiomatic code mean?
Let us say you are using Python to populate a list with numbers.
One way to do this is
nos =  for i in range(5): nos.append(i)
nos = [i for i in range(5)]
The second one is the idiomatic code. It leverages the Pythonic way of coding. Python is just an example here; every programming language has a philosophy and a method of doing things. Code that adheres to this is said to be idiomatic.
The advantage of idiomatic code is that it takes less mental work and space to understand. The first one spans a couple of lines which translates to spending more grey cells to understand what is going on. The second one is concise and to the point. You end up expending less mental bandwidth to understand the second piece of code.
Also, idiomatic code is an inside speak between people in the know; like how a society functions with social norms and conventions, programming languages and communities achieve the same through idioms and conventions. You look at idiomatic code and you know instantly what this piece of code is trying to do, it is embedded in your subconscious like muscle memory.
PS: You can populate a list in Python in a lot of different ways including using built-in library functions. Debating that is not the point of this post.