A speed brake is a device for slowing the aircraft in the air, by creating extra drag.
It uses spoilers on the wings or the fuselage, depending on the type of aircraft.
Therefore one does not wish to take off if the warning light is showing, as it is like driving a car with the handbrake on!
Perhaps it was only an indication error (the speedbrake was not really deployed), but the crew would need to stop and check it out.
I hope that this clarifies your question.
FYI, I am not an airline pilot, but a PPL with 15 hours training on an airline simulator, where the instructor demonstrated this situation - BTW it is called a rejected take off.