Some draws in Texas Hold’em poker can hit your hand almost accidentally and will be extremely difficult for your opponent to detect. For instance if you are holding 6♦ 8♠ and saw the flop for free from the big blind, you might not get especially excited by a board of 4♣ 7♠ J♥ . However your hand would improve dramatically with a 5♦ turn and there would be almost no way for your opponent to know your hand.
When you hit a “hidden draw” like this, you will often get paid off on your river bets by an opponent who thinks you are making a bluff.
It is not ordinarily good practice to be making calls with weak hands, or to be calling with weak draws. But be sure to maximise your profit if you manage to make a big hand.