Here is a puzzle for you:
Suppose you stick an electrode inside somebodies brain and listen to the activity of 1 nerve cell. Lets say that, on average, it gives out one electrical impulse (or, action potential) per second. Then its firing rate would be equal to 1 Hz, a pretty moderate value. Lets now move the electrode and eavesdrop on a different neuron. It spikes with a frequency of 0.6 Hz, so a bit lower. now suppose we repeat this process for a huge number of neurons, and then plot a histogram where the x axis shows the firing rate and the y axis represents the number of neurons who had the corresponding firing rate.
What do you think the resultant shape of this distribution would look like?
