In the barnyard, Dorothy talks about going "some place where there isn't any trouble." She asks, "Do you suppose there is such a place, Toto? There must be. It's not a place you can get to by a boat or a train. It's far, far away. Behind the moon, beyond the rain...."
She wants to go "over the rainbow", where there are no problems.
This cues up Dorothy for singing her beloved song, "Somewhere Over The Rainbow", while she strolls from a bale of hay (on which she leans back), to an old wheel (that she pulls), to a discarded piece of farm machinery (on which she and Toto sit).
"Somewhere Over The Rainbow" by Judy Garland as Dorothy in The Wizard of Oz.