Ice Cream mix tends to freeze at about -2.7 C, the container which is holding the ice cream must cool to, or below that temperature. Ice made from regular H2O isn’t enough to cool the container on its own because when ice melts, the temperature reaches 0 C, which is above the ice cream freezing point.
NaCl and H2O have a lower freezing point than regular water, and when NaCl and H2O are combined, a freezing point depression (a colligative property) happens, which is when the freezing point of a solvent is lowered. The salt dissolves the ices surface, and an endothermic reaction occurs. When H2O and NaCl are combined (being polar compounds) the NaCl separates into Na+ and Cl-, which is called dissociation. NaCl dissolves into H2O, which lowers the freezing point of the water, making it an ideal temperature to cool ice cream, which is called an endothermic phase change.