Prior to the adoption of Cascading Style Sheets (CSS), the spacer GIF was a transparent image, often used to control blank space within a web page, that can be resized according to the width and height dimensions it is given. The reason a spacer GIF is invisible is so that an HTML developer can create a table cell and fill the background with a specific color that can be viewed through the transparent spacer GIF. For instance, a developer seeking to create a square blue box 500 pixels on a side could use a separate blue 500×500 graphic at the expense of additional bandwidth. Instead, the developer can specify the table cell background color and specify the dimensions of a pre-existing spacer GIF.
Read more about Spacer GIF: History