In the winter, it keeps soil a little warmer, which limits chilling stress on tender plants.
When temperatures rise, it insulates the soil, keeping it slightly cooler.
It may help to think of mulch as similar to insulation in the attic.
Mulch can be almost any substance that lays on the surface of the soil, but some things are more beneficial and practical than others.
Read more about the importance of mulch in your garden!