Consider the simple black and white image below:
Let it represent the edge of a mask. Its Histogram will look like this (below):
A histogram is a graphical representation of data. PS's histogram charts the distribution of the light values of an image. The histogram above shows a spike at white, a spike at black and nothing in between. Now lets blur the image (below):
The histogram (below) shows that the blur has resulted in a full set of grey values.
As well as charting data, the histogram can also be used to move the data around. If the black and grey values are moved towards the right, everything from light grey to dark gray will be converted to black. This will give us this (below):