Cold fusion is a type of nuclear reaction that would occur at room temperature. Cold fusion is compared with the hot fusion that takes place within the stars, under tremendous pressure at temperatures of millions of degrees and distinguished from catalyzed fusion. Currently, no accepted theoretical model would allow cold fusion to occur.
If cold fusion were possible, it would be an immense source of energy, but the problem is that there is currently no theoretical framework in which cold fusion would work. The general scientific consensus is that it can't work, but that hasn't stopped a minority of scientists still researching the subject.