fread(pImg + i * bmh.biWidth, 1, bmh.biWidth, fp);

This code reads one row of pixel data from a BMP image file.

The function `fread()` is a standard C library function that reads data from a file.

The parameters of the function are:

- `pImg` - a pointer to a buffer where the data will be stored
- `i` - the size of each element to be read, in bytes (in this case, 1 byte per pixel)
- `bmh.biWidth` - the number of elements to be read (in this case, the width of the image in pixels)
- `fp` - a pointer to the file to be read

So this line of code reads one row of pixel data from the BMP file pointed to by `fp`, and stores it in the buffer pointed to by `pImg`. The size of the row is `bmh.biWidth` pixels, and each pixel is 1 byte in size.