In a survival scenario, snow should be boiled before drinking to ensure it is clear of contaminants.

The mod Snow is water [B41.74 Legacy] (or its variations) fixes a logical gap in the base game: by default, rain barrels do not collect water during winter when it snows.

There are "Rainbarrel only" versions and "Project Realism" versions that focus on melting snow for drinking water. General Concept: Melting Snow for Water

Snow typically fills containers at a slower rate (often 50% ) compared to rain.

The phrase you are referring to typically points to a popular community-created mod for the survival game . Project Zomboid Mod: "Snow is Water"

Outside of gaming, "Snow is water in a barrel" refers to the practical survival technique of collecting snow to melt for fresh water.