What is the snow globe liquid?

This depends on how old the snow globe is, and where it was made.

Very old snow globes were originally made with water. These may develop bacteria and mold over time; the liquid may also yellow depending on what the objects are inside the globe.

Snow globes made overseas are quite often filled with a combination of anti-freeze and water to prevent freezing. Anti-freeze is VERY toxic to pets and humans, so if you own a snow globe made in China, Hong Kong and the like, assume it is dangerous and don't display in an area where it could be broken and have a dog lap it up.

Snow globes made in the last ten years in the United States (by a manufacturer, not an individual) are likely enclosing a food-grade, safe liquid. Nobody recommends you break a snow globe and drink it, but it's good to know that U.S.-made custom-made snow globes are non-toxic.

An individual artist who makes their own globes may use any of the above, but often will try a combination of water and glycerin to slow down the floating of confetti and/or "snow" bits.