The best way to understand it is by using the help or doc commands to look up each function name. At the top of each page you'll see a brief description of the function, inputs and outputs.
Here's a very general description of each line
% CHOOSE A FILE
[filename,pathname] = uigetfile('*.*','Choose the input image');
% READ THE IMAGE FILE IN AS IMAGE DATA
im = imread([pathname,filename]);
% DETERMINE THE SCALE OF THE IMAGE DATA
% (max(size(...)) returns the images maximum dimension
% THE IMAGE DATA (im) IS AN nXmX3 ARRAY OF R-G-B- COLOR VALUES
% BUT THIS LINE ONLY REFERENCES THE "R" VALUES
scale = 600/(max(size(im(:,:,1))));
% SCALE THE IMAGE
im = imresize(im,scale*size(im(:,:,1)));
% GET THE NEW IMAGE SIZE
[m,n,~] = size(im);
You can use imshow(im) to see the image.
