What are bracket pairings?

D3 1st v D4 4th
D1 3rd v D2 2nd
D1 2nd v D2 3rd
D3 4th v D4 1st
D2 1st v D1 4th
D4 3rd v D3 2nd
D4 2nd v D3 3rd
D2 4th v D1 1st