The word relative means in relation to.

Therefore, relative speed would be the speed in comparison with another object.

Let's say you are traveling at 30 miles per hour on the free way and there's another going in the same direction as you at 20 miles per hour, then your relative speed with respect to the car is 10 mph.

However, at the same time your relative speed with respect to another car traveling in the opposite direction at 20mph would be 50mph.

Another example, would be let's say you are walking on a train that is traveling at 100mph at 5mph in the same direction as the train. Then your speed with respect to the ground would be 105mph. Here 5mph would be the relative speed while 105mph would be your absolute speed.