 A lot of boards, like the Feather microcontroller here, feature 0.1-inch spaced, solderable holes that make breadboarding easy. Solder header pins on, plug it in, and you've got a strong connection. If you want to attach accessories to your board, like this feather wing, solder socket headers to the feather instead. Solder pins to the feather wing, and attach. What's that? Plug a microcontroller into a breadboard and attach an accessory? Well, you're in luck because stacking headers are a thing. They're a combo of plug and socket headers, essentially socket headers with very long pins. Align them for soldering by plugging a matching daughter board into the sockets. Solder the base of each long pin to a pad. And now you're double stacking electronics. Nice.