Look at
this page using IE - then look at the same page using Mozilla or another pbrwser - you will see that the Image that is repeated on the page has a grey border where it should be transparent until the "fix" is applied...
This hack provides a way to apply that fix to your forum without having to hack each location where the PNG-24bit transparent images appear.
If you want to know more about how mouch work it is, there is a style released on vBt that has this fix applied throuhgout it manually - the coder who wrote it would be able to explain how much more work it is.
The reason to do this is because GIF transpararency is good, but the quality of a GIF is terrible unless the filesize if huge - PNG24 files can be tiny filesizes while being large images with HQ transparency and no loss in the compression.
More than one sentence, but there you go.