valve trouble....help me
 

i have a 1985 ford f150. it has the straight 6 in it. i took the valve cover off to clean. while cleaning the sludge off the head i found a small metel circle peice ( about 1/4 inch ) sitting behind a hole. there is no crack or anytype of fracture in the circle peice. i disscoverd that it came out of the spot between the top of the valve stem and the rocker arm. the motor runs and drives.

is that little peice apart of the valve?
does that peice just sit ontop of the valve stem?
how do i fix it?
is it hard to fix?
is it a bid deal or not?