This means that your brake fluid is low. Instead of just adding more fluid you should bleed your brakes. Brake fluid is only good for 30k miles tops. Brake issues are a matter of critical safety and importance and must be addressed immediately...
Mine does the same from time to time. There is a small fluid leak coming from the top the left rear caliper where the brake line feeds into it. Very small leak though, havnt added fluid in pobaly 2 weeks or so.
This means that your brake fluid is low. Instead of just adding more fluid you should bleed your brakes. Brake fluid is only good for 30k miles tops. Brake issues are a matter of critical safety and importance and must be addressed immediately...