Isaiah explains this metaphor in verse 15. The "head," the part of an animal a person would want to be, is "the leader and the noble man," and the "tail," the dirty part of the animal, is "the prophet who teaches lies."
The "palm branch" grows high on the tree and is a metaphor for people who are important and rule others. The "reed" grows in shallow water and stands for people who are poor and unimportant and are ruled by others.