The idea behind this is astonishingly simple. Instead of removing a straight column (row) of pixels to reduce the width (height), a so-called vertical (horizontal) seam is removed. The seam is a connected path of low energy pixels in the image. Thus, removing a seam does not alter the high energy content, which is usually semantically useful.
An image is analyzed and seams are ordered from low to high energy. The seams with lowest energy are removed first for reducing the size of the image, and subsequent seams are removed to reach the desired image size. The image can also be expanded by adding in extra pixels in these seam locations with values as the average of the neighbours. The video below throws light on how these seams look and shows nice applications.