What is the difference between binary and gray scale images?
Normally, there are 256 levels of gray. But a binary image has only 2: black and white. So, instead of a table of numbers from 0 to 255, this is a table of 0's and 1's.
Thresholding turns gray scale images into binary ones.
The analysis of gray scale images has to be more subtle, even though the idea is the same.