This is a development board for NXP KW41Z with an onboard 3-24 V input voltage regulator, PCB trace antenna, and JTAG/SWD header. The form factor and pinout match that of an Arduino-pro-mini.
The MKW41Z256VHT4 has a Cortex-M0+ with 256KB flash, 64KB ram, and a dual-mac 2.4GHz transceiver that supports 802.15.4 and BLE.
On the rear side is a footprint to add SMTU2032-LF for CR2032 battery operation. The battery life can be up to several years if you do your power management properly and don't need to send more than a few bytes of data per second.
These come with no bootloader and must be flashed over SWD. They'll come programmed with this working RIOT example, which looks for a 6lowpan gateway to connect to on channel 26 and runs a telnet server you can play around with.
RIOT support for this platform is mostly complete however you'll need some PRs that haven't been merged into master yet. I'd recommend getting started with this branch as it includes all the required PRs for radio support and power management. It has some extra stuff too, but it's the branch I keep most up to date right now.
I'm planning to change the next revision of these to replace the 10-pin SWD header with a 3 or 4 pin .1 inch header with GND, SWDCLK, SWDIO, and potentially VCC, as I think the future of most JTAGing for open hardware is probably a raspberry pi and some gpio pins, and anyway it's annoying to have that big connector on every board when it might only ever be used once. Leave a comment if you have an opinion on this.