According to the manual, this (a red light that says BRAKE/(exclamation point with a circle around it) can mean one of two things:
1. If it lights up when you have the parking brake engaged, then goes off when you disengage, there is something wrong with your brake system. Take it in for inspection immediately.
2. If it lights up when you’re driving, the brake fluid is low. Either you have a leak, or your brake pads are worn.
Check for more details in the manual for both of these cases.
I think #2 is your case based on your description. I should note that the manual says that if your brake fluid is below the “min” level, have the car towed to the dealer. Personally, I think that’s probably overly cautious unless you can see a leak or the brake fluid level is extremely low and dropping.
In my experience, usually it means my brake pads are worn and I need get new pads installed. I tried topping off the brake fluid once with on old Outback of mine awhile ago, but ultimately that means more work for the mechanic; when the new brake pads are installed, the fluid in the reservoir will go up, and they may have to remove the extra fluid.