Instead of a prayer to a god, the speaker addresses his own legs, as if they were restless animals that need calming and then sending in the right direction. The poem is only a few words long, and that smallness is part of its charm. The title calls it a "charm," which suggests a spell or a bit of folk magic, something you say aloud to make a difficult thing happen.
The difficulty is busyness: the pull of tasks and errands that keep a person moving long after they should stop. The speaker asks his legs first to "be still," then to "be off" to his wife and to bed. Those two commands seem to pull against each other, since stillness and going somewhere are opposites, but they resolve in the last word. Bed is where the movement ends.
The mood is tender and a little funny. There is tiredness in it, but also longing. The speaker wants rest, and he wants to be near his wife.