This structure is used to describe a block of image data.   The image data is stored in the Data property with the Width and Height properties describing how many pixels the image has on each dimension.   

The BytesPerRow property describes how many bytes are used on each row of pixels.   This is often referred to as the stride of the image.   It does not necessarily have to match the width in pixels, it can often be set to a different value to ensure that each image row starts in an aligned memory address (this is typically done to improve performance as CPUs are able to perform aligned fetches from memory faster than unaligned ones).