The term holy grail, as was used by Patricia and Lucy, means an object or goal that is sought after for its great significance.
Why is the Pee Wee Reese card significant? Well, it isn't his rookie card. The 1941 Play Ball card # 54 is considered to be his rookie card. This one was issued 12 years later. I think that it is the whole package. The colors, the action, the framing, the clouds. A recent copy of this went for $1,100 in the last Robert Edward Auctions sale. It was PSA Graded NM-MT 8. I know that I can't afford it.
But here's a card that is similar...
1991 Topps Walt Weiss (card # 455)
No, Walt isn't in the Hall of Fame and probably never will be. But you can pick this one up for $1.10 on ebay. Free shipping. You could probably pick it up for a lot less from your local card store.
Not "the holy grail of mid-air cards," but it is a nice one.