Frequency
One way to visualize this is if there were two trains traveling at the same speed, but the size of the train cars was smaller on one train than the other. If someone picked something that was not moving, like a signpost, and then counted how many train cars passed the sign post in one second for each train, they would know the frequency of cars passing in each train. The number and frequency of train cars passing the sign post would be different, because the train with smaller train cars would have more train cars passing the sign post in a second than the train with larger train cars. Knowing how many cars passed the sign post in one second, and knowing the speed of the train, one could figure out mathematically the size of each train car for each train.