Remember the ALT attribute in IMG tags? The SUMMARY attribute in TABLE tags serve a similar purpose, in that it will provide a description of the table to users of non-visual browsers. Unlike the ALT tag, Explorer 4.x, Communicator 4.x and lower browsers do not use the SUMMARY tag to display pop-up hints when the mouse is paused over the table.
NOTE: You may not see a visual change by adding the SUMMARY attribute if you're using a graphical browser. However, HTML authors concerned about their content being accessible in different mediums and users with disabilities, for example, are advised to use provide a summary of the table.