In the holler

"Make yourself comfortable," he says at his farm in the holler.

What's "in the holler" supposed to mean here?

A holler is a hollows. Here's what the OED says, s.v. hollow. "2. spec. A depression on the earth's surface; a place or tract below the general level or surrounded by heights; a valley, a basin."

Hollows, especially when they are called hollers, are primarily found along the Appalachisn Mountain chain in the souther U.S.
SS

Holler is a phonetic spelling of an Appalachian pronunciation of hollow. A village in West Virginia, for example, might identify itself as a holler, after the surrounding geography.