Like the cold of snow in the time of harvest is a faithful messenger to those who send him; he refreshes the soul of his masters. (Proverbs 25:13)
Harvest time is during summer and can be hot in Israel. In Bible times, there were no refrigerators so it would have been difficult to get a cold drink while working in the harvest fields. If someone could produce a drink as cold as snow it would have been very welcome and refreshing. In the same way, a faithful messenger is refreshing to his master.
Some modern versions make the simile clearer:
Faithful messengers are as refreshing as snow in the heat of summer. They revive the spirit of their employer. (Prov 25:13 NLT)
A messenger you can trust is just as refreshing as cool water in summer. (Pro 25:13 CEV)
Reliable friends who do what they say are like cool drinks in sweltering heat–refreshing! (Prov 25:13 MSG)