Idiomatic in a programming language means pretty much what it means in any other language: how would someone fluent in the language express it? In a programming context, that may also correspond to the most performant or otherwise "best" expression, but sometimes it's just a commonly used style or phrase.

A programming language idiom is a pattern or technique that you expect your readers to know, even if it isn't obvious when encountered for the first time. Here's an idiom:

    for (int i=0; i < 10; i++)

We all instantly know this means loop 10 times, from 0 through 9. But on first read it takes a little work to figure out what's happening.

I was struggling with that too initially - replace the word with 'standard' or 'natural' and it retains the same meaning.