It is not clear from your quesiton what exactly the dentist has done. Normally after extraction gauge piece is given as pressure pack over the extraction site to control the bleeding and it has to be removed after min 30 minutes. But sometimes if the bleeding is not controlled and surgical extraction is done then some haemostatic agents are used to control the bleeding. They are put in the extraction socket and stitch is given over them but that you donot have to remove and they dissolve itself. You can contact your dentist and find out about the plastic piece. for more info on dry socket which is most common complication after tooth extraction you can read this article... http://www.identalhub.com/dental-what-is-dry-socket-879.aspx