Sublime Text is a notepad-like application that lets you write your code, highlight certain parts using its various syntax detecting algorithms and so on. One of the best and simplest coding tools ever available to Mac users. On the downside, Atom is fairly memory-intensive, particularly when handling large files, so be sure that your computer is up to the task RAM-wise before jumping in. Beyond that, Atom is filled with quality-of-life features like autocomplete, a robust search tool and file browser, and a powerful package manager. You can work in the universal hosting service GitHub from directly within Atom too, doing every function from creating new branches to pushing and pulling code requests.
It has a great collaboration tool in the form of Teletype, which lets you seamlessly work on code with fellow users.
Whether you want to tweak source code or play around with packages, you can do so here. Atom is a fantastic code editor for users of all skill levels.