Why does my Rpms drop when I drive the truck in gear?

Why does my Rpms drop when I drive the truck in gear? So I got my truck tuned about 2 days ago and it was running fine but I noticed the rpms where dropping to zero when I came to a complete stop same problem when idling it would be at 500 rpms and drop to zero truck does not shut off at all any idea on what it can be ?