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 and saw the flop for free from the big blind, you might not get especially excited by a board of . However your hand would improve dramatically with a 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.