Why A comparator with no hysteresis switches the instant the input crosses a single threshold, so a noisy signal hovering near that point makes the output chatter back and forth. Hysteresis adds a small amount of positive feedback so that the threshold moves after each transition: once the output goes high, the input must fall to a lower level before it switches back. Those two separate trip points mean noise smaller than the hysteresis gap cannot flip the output, giving one clean transition.
Watch out The claim about increased sensitivity is backwards. Hysteresis deliberately makes the comparator less sensitive near the switching point, which is exactly what buys the noise immunity.
Hysteresis = two thresholds with a dead zone between them, so noise can't make the output chatter.