What is a bin in a histogram?
The "bin" in a histogram is the choice of unit and spacing on the X-axis.
All the data in a probability distribution represented visually by a histogram is filled into the corresponding bins. The height of each bin is a measurement of the frequency with which data appears inside the range of that bin in the distribution.
By way of example, in this sample histogram below, each bar ascending upwards from the X-axis is a single bin.
And in the bin from Height 75 to Height 80, there are 10 data points (in this case, there are 10 Cherry Trees of a height between 75 and 80 feet).
Source: Wikipedia page on Histogram