“Garter snake” is a traditional American term for small harmless snakes with stripes running lengthwise along their bodies, resembling old-fashioned garters. It is more broadly used for all manner of small non-venomous snakes. Many folks don’t get the allusion, and call them “gardener snakes” instead. Although you may find these little critters in your yard, they are unlikely to do much gardening. For that you need earthworms.