i want to know about the function " [r, c] = find (image)" in detail

hello ... am working on OCR background for converting document image to speech.So, i got some source code from matlab file exchange " http://in.mathworks.com/matlabcentral/fileexchange/18169-optical-character-recognition--ocr-/content/OCR/lines.m " and yeah it helped me lot in my project, but the problem is am not able to understand the function 2 lines from the code

[f c]=find(img_in);
img_out=img_in(min(f):max(f),min(c):max(c))

can anyone explain how exactly these two lines works on image (i.e) what are f & c ,img_out=img_in(min(f):max(f),min(c):max(c)) . !