What is SiLabsUSBDriver.kext? What is SiLabsUSBDriver.kext? Is this part of the official Yosemite system and if so, how do you know? Where can someone go to find this out from Apple directly? Thanks.
It's the kernel extension for the Silicon Labs USB to Serial driver.

It is not a part of OS X Yosemite as it's not included in the Install OS X Yosemite.app bundle.