Advanced Host Controller Interface
http://en.wikipedia.org/wiki/Advanced_Host_Controller_Interface

AHCI ON => higher/better HDD (or SSD) performance.

If you did not enable AHCI prior to the installation of Windows in order for it to work you have to enable it and reinstall (clean install) Windows.
There are ways to enable AHCI on a drive which has Windows allready installed on it, but sometimes it does not work that well or even gives lower performance.

ACHI is essentially a command and reply workflow that assists in not overflowing the transmission bus with commands, which slows data throughput.

Figure that on standard ATA command workflows, commands, replies, and data (in and out) all exist on the same channel, so it can get rather cluttered. We lived with this because data was so slow from older magnetic disk drives that changing the command arrangement wouldn't have helped us speed up. The clutter just filled in the spots where we were waiting for the drive to catch up.

Newer drives with high data density (fast), hybrid SSD/magnetic drives (faster), and true SSD drives (fastest) are pushing beyond what ATA was designed for, and ACHI gives us the newer, more organized command and data method to keep the data flowing at fast rates. Many recent SSD devices will actually crash if ACHI isn't enabled as their controller literally freaks out under ATA and can't organize quickly enough.