The function of " distinct on"

A B C D
1 1 2 3
1 1 2 4
1 1 2 5

select distinct on (A,B,C) A,D,C,D from hoo order by A,B,C,D
output

A B C D
1 1 2 3


select distinct on (A,B,C) A,D,C,D from hoo order by A,B,C,D desc;
A B C D
1 1 2 5