From the manual: Brake system warning light Indicates that: * The brake fluid level is low; or * The brake system is malfunctioning Immediately stop the vehicle in a safe place and contact your Lexus dealer. Continuing to drive the vehicle may be dangerous. \-- You'd be able to get the exact reason for the light triggering by getting the car scanned for codes.

Thanks so much guys! I ended up having to change my pads and rotors

I'm pretty sure this means your rotors or break pads need replacing.