PIE stands for “Progressive Internet Explorer”. With PIE you don’t have to wait any longer to use the CSS3 border-radius property. A lot of browser already support it out of the box, but Internet Explorer only since version 9.
To also have support in Internet Explorer 6, 7 and 8 use PIE! There are two options for using it. With a htc behaviour file or with JavaScript.
Read more about it on the official website: http://css3pie.com/