What does a negative velocity mean?

To understand this question, it is important to know that velocity has both a direction and a magnitude. The magnitude can be expressed by the unit m/s, however the direction is expressed by a vector. To distinguish between two directions in velocity, you can have a positive and a negative vector: vectors that point in the opposite direction. Therefore, a negative velocity means that the direction of that velocity is opposite to the other velocity (with the positive value).