An image can be very large, much larger than the available memory, so you can’t just access pixels with a pointer *.

Instead, you read pixels from an image with a region. This is a rectangular sub-area of an image. In C, the API looks like:

VipsImage *image = vips_image_new_from_file( filename, NULL );
VipsRegion *region = vips_region_new( image );

// ask for a 100x100 pixel region at 0x0 (top left)
VipsRect r = { left: 0, top: 0, width: 100, height: 100 };
if( vips_region_prepare( region, &r ) ) 
  vips_error( ... );

// get a pointer to the pixel at x, y, where x, y must
// be within the region

// as long as you stay within the valid area for the region,
// you can address pixels with regular pointer arithmetic

// compile with -DDEBUG and the macro will check bounds for you

// add VIPS_REGION_LSKIP() to move down a line
VipsPel *pixel = VIPS_REGION_ADDR( region, x, y );

// you can call vips_region_prepare() many times

// everything in libvips is a GObject ... when you're done, 
// just free with
g_object_unref( region );

The action that vips_region_prepare() takes varies with the type of image. If the image is a file on disc, for example, then VIPS will arrange for a section of the file to be read in.

(* there is an image access mode where you can just use a pointer, but it’s rarely used)