Other people's jobs
When you’re just starting your career, most of your professional development revolves around skills that are part of the job requirements.
If you want to be a programmer, you need to learn how to program. And knowing more about programming makes you a lot more effective.
However, once you become good at the technical skills, you may realize that acquiring more of them might not matter as much anymore. The advice I’m about to give you sounds counterintuitive at first, but trust me, it’s very impactful:
Learn how to do other people’s jobs.
Learn what people you work with do. Become interested in how they think and what they value.
This will give you perspective. It will help you see the big picture, communicate better and make better decisions. And it will make you and people around you more productive.
P.S. If your goal is to get promoted, learning your boss’s job is very effective as well.