A histogram provides a visual representation of the variance in a specific process variable.  The variable which is selected is usually directly associated with an important process goal such as safety, profit, conversion, efficiency et cetera.  The histogram is a representation of the amount of time (the fraction of the total number of data points) at which the selected variable had a specific value.

From the below histogram, we can see that the reactor pressure was between 336.5 and 337.5 kPa for 25% of the time.