Headers are like the duct tape of electronics. They're great for connecting things together, using breadboards to build up a circuit, soldering to perf-boards, or preparing boards for plug-in components like servos or sensors.
Using headers with your microcontroller can make your life much easier, but soldering them on can be a little intimidating to beginners. There are a lot of ways to get it wrong, and once a header is soldered onto a board it's difficult or impossible to get it off again.
This guide will teach an easy and reliable way to attach your headers, with tips for getting them on straight and accurate, so you can move on with your project with confidence. It will also give advice about pitfalls to avoid, and what to do when you get them on wrong.
You might also enjoy a look through Adafruit's Guide to Excellent Soldering, if you're just getting started.