The word ‘farmhouse’ comes in the poem in the context of the horse feeling it strange to stop by the snowy woods without a farmhouse nearby —

My little horse must think it queer
To stop without a farmhouse near

So, keeping the context in mind, on a symbolic level it means ‘our familiar world’, our own known world outside which we feel uneasy. As suggested in the line quoted above, the horse finds it awkward as his master stops at an unusual location away from his familiar destination.