A face card is one of the non-numeric playing cards that comes after ‘ten’, which usually depicts a figure – the Jack, Queen and King. These are also sometimes known as the ‘court cards’. In a traditional pack of cards the figure on the face card is mirrored upside down on the lower half of the card. This means that whichever way the card is held, it is always the right way up.
Lennie is talking about playing cards, but his comment could also apply to Of Mice and Men, which in many ways ends in a similar place to its starting point – same location, similar trouble, repeated words and ideas, etc. This gives the story a sense of inevitability. George and Lennie can dream big, but in the end they will finish up right back where they started.