Let’s see the most common example. You are in the late position and your hand is: A, 2, 7, 7 and except you, two more players enter the flop. The flop is as follows: 3, 8, and Q of different suits. The opponent from the early position places a bet, the opponent in the middle position calls. You also call, since you have chances to get nuts low on the turn or the river. The turn is 5 of the fourth suit. The aggressive player in the early position places a bet and the opponent in the middle position who called on the flop now raises.
So, how can we interpret this raise? As we can see, the turn did not change anything. Of course, theoretically, it was possible the turn to give the raiser a set of 5s, which will satisfy you in the best possible way. That’s because then you’ll have two opponents with strong high combinations and you’ll hold nuts low which will bring you half of the pot formed mainly with their money. The probability of this situation should be considered but it is very low. It is much more possible, that the opponent in the middle position has already formed nuts low. In this case, he will try to increase the pot at your money expense since he does not expect to fail (he does not know that you also have formed nuts low). If that is the case, you don’t have any choice but to call and regret that you are not going to make a profit, only losses.
From this example, we can see that your opponent did not play right, And even the late position did not save you from loss but anyway the loss was minimal. Since you suggested that the raiser had also hunted low, you called and if the river wouldn’t change anything, you’ll have no other choice but to call again.