What does it mean for a card to be scarce

In this video I take a look at a few cards that have varying degrees of scarcity. What does it mean to be scarce? Is a 70 year old card automatically scarce? Using the 1954 Bowman and Propagandas Montiel sets as a basis for discussion, we talk pop reports and real scarcity. How do you collect? Do you value a card that is truly scarce?