The Terminal Block Breakout FeatherWing kit is like the Golden Eagle of prototyping FeatherWings (eg. majestic, powerful, good-looking).
To start, you get a nice prototyping area underneath your Feather, with extra pads for ground, 3.3V and SDA/SCL. Not one to stop there, Adafruit expanded the PCB out to 2" x 2.5" with 3.5mm pitch terminal blocks down each side. There's also four mounting holes so you can attach the breakout to your enclosure or project.
This product works with all Feathers! The terminal blocks allow you to connect to any of the external Feather pins, great for wiring temporary or permanent installations. Adafruit also give you a few extra terminal block pins for ground and 3.3V connections since those are so useful. The terminal blocks included with your product may be blue or black.
Finally, there's a slide switch, which connects the EN pin to ground when in the 'off' position, cutting off the 3.3V regulator. Note that the FONA Feather uses both VBat and 3.3V as power supplies so you wont be able to fully turn off the FONA Feather with this switch.
Note: As of Thursday, December 15th 2016, this product now comes fully assembled! Plug in your Feather and you're ready to go immediately.
Some folks have noted that this breakout doesn't play well with Particle Boron, please add a 10K resistor on the EN line to make it work better.
As of Feb 5, 2020 - Adafruit have made the trace to the enable pin thicker, and added a ground plane which helps keep the enable line clean some high-noise environments.