It indicates that the product of the size of f in dimensions 3 through ndims(f) is 3. Most of the time, it means that f is a 3-dimensional array with three pages.
For example if we were looking at a notebook that is 7 inches tall and 5 inches wide with 100 sheets, we could say that its size is [7 5 100]. [Yes, I know I'm mixing units and I'm ignoring the covers. Bear with me for the sake of analogy.] If you computed:
[row col dim] = size(theNotebook)
row would be 7, col would be 5, and dim would be 100. If you had torn out 97 of the pages then dim would be 3; only three pages are left.
