Quick answer
A messenger you can trust feels like cold in the heat. The verse is about relief arriving when it was needed and not expected.
Read both lines
Harvest fell in the hottest part of the year, and snow was not something that happened then, so the image is either a chilled drink carried down from the heights or simply the impossible thing, which is the point either way. Relief at exactly the moment the work is hardest.
The third line explains the comparison rather than extending it: he refreshes the nephesh of his masters, restores the life of the people who sent him. That is a large claim for a job description, and it lands because everyone who has waited on news knows the alternative. This book cares a great deal about messengers because in that world a message was a person, and an unreliable one meant your decision was made on a lie.
Related: Proverbs 13:17 · Proverbs 26:6 · Proverbs 25:25 · Proverbs 10:26
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.