The statement tells us that the speed is constant. Then, the equation d = 65t, is the equation of a line, whose slope is m = 65. This equation tells us that for each unit of time in hours the distance "d" increases by a constant factor of 65.
This allows to conclude that the speed of the car is 65 miles / h.
Therefore, in this situation the number 65 that appears in the equation represents the constant speed at which the car travels.
It tells us that every hour the car travels 65 miles