Muddiest Point for Week 4

I still do not understand the question I asked at the lab this week. For example, when I imported the "book.xlsx" from assignment #2, the bookid column's indexed showed: "Yes (Duplicates OK)". However, other columns were all showed: "No".

I don't know what does this mean, and what is the difference between "Yes (Duplicates OK)", "Yes (No Duplicates)", and "No". I just want to figure out is this make any differences.