When salt lands on ice, it dissolves into the thin film of water always present on the ice surface, forming a brine. This brine has a lower freezing point than plain water, meaning the mixture needs a colder temperature than 0 degrees Celsius before it can stay frozen. In practical terms, this pushes the temperature at which the ice starts to melt further down, so instead of melting readily, salted ice can hold its frozen state at temperatures where plain ice would already be turning to water, effectively slowing down the melting.