OpenCV. Processing external byte buffer

We have such an example from openCV documentation:

Mat img(height, width, CV_8UC3, pixels, step);
GaussianBlur(img, img, Size(7,7), 1.5, 1.5);

What does it mean? 

Lets say I have an image img:

typedef unsigned char BYTE;
BYTE * img = new BYTE[ w * h ];

Lets assume the image is scanned line by line. So

img[ 10 * w + 3 ] is a 3-rd pixel in 10-line of image.

Then I import it in openCV is such a way:

Mat img(h, w, CV_8UC1, dmDst, w);

last argument is a pitch or stride.