Strictly speaking speed is just a number, it is
velocity that could be positive or negative.
Velocity is a vector which means it has
direction as well as size.
A negative velocity just means that you are traveling in the
opposite direction.
For example:
An ant crawling across this page from left to
right might have a velocity of + 0.1 m/s
If it crawled from right to left at the same speed
its velocity would be – 0.1 m/s.
This idea is pretty important. Imagine the different
result of a car at a set of traffic lights moving off at - 2 m/s (lets say - represents backwards)
rather than at + 2 m/s (forwards).