OpenHAL refers to code that aims to replace the proprietary, closed-source HAL. The code is based on ar5k, part of OpenBSD's 'ath' driver for Atheros wireless cards. ar5k is the component of the driver that talks directly to the hardware, it was produced thanks to reverse engineering efforts done by Reyk Floeter. It's an important asset on the way towards a free (as in freedom) driver for Atheros devices.