You may be thinking of the situation where I, riding in the back of a bus at 40 mph, throw a ball forward at, relative to me and the bus, 50 mph. Relative to you, standing at the side or the road, the ball is traveling at 50+ 40= 90 mph (approximately).
But the basic concept to relativity is that the speed of light is the same for all observers. If, instead of throwing a ball, I pointed a flashlight forward, you would see the light still moving at speed c, not at c+ 40. If I were on a spaceship moving with speed v, you would see light moving at c, not c+ v. If v were greater than c you would see the light moving backward relative to the rocket because the rocket is faster. (0f course, relativity says it is impossible for an object to move faster than light so this whole scenrio is fishy!)
(I said, above, that the ball's speed, relative to you on the side of the road, would be approximately "50+ 40= 90 mph". The reason for the "approximately" is that the Gallilean "addition of velocities", v_1+ v_2, does not exactly apply even for slower moving objects. The relativistic formula is
\frac{u+ v}{1+ \frac{uv}{c^2}}
Since the denominator is larger than 1 for any non-zero u and v, the resultant speed is slightly less than "u+ v". Of course, c is so much larger than "40 mph" or "50 mph" (about 186000 miles per second which is 669600000 miles per second, uv/c^2= (40)(50)/(669600000)^2 would be about 0.00000000000000446) that would be indistinguishable from 90 mph (89.999999999999598540614843079346 mph). But with v equal to c, that would become (u+ c)/(1+ u/c)= (uc+ c^2)/(c+ u)= c(u+ c)/(c+ u)= c.)
