According to my owner's manual, I believe it means that your brake pads are low and will need replacing soon.
I'm new to Lexus, but I did register on www.lexus.com in the owner's site as an owner. There, if you navigate around, you will find a link to order a FREE replacement owner's manual. Mine came in just 5 or so days just this week!!!!
its a 93 LS400 however they are probably all the same. thank you