On its own, salt cannot melt ice; it must be combined with water to lower the freezing point of the ice onto which the salt water solution is poured. However, ice can form again if the salt water solution reaches a temperature low enough to stop the melting process.
The freezing point of water is 32 degrees Fahrenheit, but by adding salt, the molecules tend to stick together, which prevents heat loss. The now-melted ice and salt water solution must reach a colder temperature to refreeze. This will occur if the temperature of the solution reaches approximately 28.4 degrees Fahrenheit, which is the freezing point of salt water.