What does it mean when the handbrake and stop symbol flash on the Renault megane dashboard?

It means dont drive your car. There would be a fault within the braking system. It could be simply a case of topping up brake fluid. Use correct grade, the megane scenic uses dot4, I assume the normal megane does too, check tho, it will be imprinted on reservoir cap. It could also be the hand brake sensor faulty. Also check your brake pads and shoes, discs/drums etc.