Elementary Statistics (12th Edition)

$P(A|B)$ means the probability of A happening if B has happened, hence $P(D|R)$ is the probability of the second selected senator being a Democrat, knowing that the first one was a Republican. Hence, because the sample was done without replacement, then if the first selected senator was also a Democrat, then for the second choice there would be less Democrats left, hence their probability would be lower. Hence it is dependent.