What does low POP mean in regards to baseball cards?

Answer 1

PSA Population Report

POP is a term used with PSA graded cards. A Pop Report, short for PSA Population Report is the publication that reports the graded population of any particular PSA graded card. Cards with a low POP in a particular grade rating will have a higher value than a card that has a high POP with the same grade. The population data is updated each day as more and more items are submitted to PSA.