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!!!!
I had the same warning "a red circle with 2 broken lines on each side came on while driving", I suspected that brake pads are low because it had been 4-5 years since that last replace.
I lost my owner's manual, thanks to the post above I had been able to order a FREE replacement owner's manual from Lexus.com.
Great site !!!