This card is from one (01) of several 'Chase Sets,' that came with the 2001 Topps NFL 'Heritage' base set -- the base set paying tribute to the classic 1956 Topps NFL base set.
Of course, in 1956, there were no Chase Sets, but several appeared for 2001.
One of best, and most unusual Chase Sets, was the 'Souvenir Seating' set. Each card paid tribute to a Hall-of-Fame player, along with (supposedly) a tiny slice, of one of the seats, from the actual stadium in which the ballplayer had performed.
This card is #S-5 ... Johnny Unitas ... Baltimore's Memorial Stadium.