bought 1GB memory stick....

and with package there was a Memory Stick Duo Adapter that looked like this

http://www.microanvika.com/products/SON57201.gif

what is it for?

Almost all new PC's nowadays have memory stick inputs along with earphone inputs, usb inputs, etc in front of the computer tower.

They were/are designed for the original Memory Stick in mind. If you try to stick your MS Duo in a memory stick input on one of those PC's you will get it stuck inside of there. That adapter is to put your MS Duo inside of that before you insert into your PC, so that dosent happen.