Back is a feature which characterizes sounds that are produced by retracting the body of the tongue from its neutral position.
The vowel [u] differs from [i] in that [u] is characterized by [+back] and [i] by [-back].
Chomsky, Naom A. & Halle, M. 1968. The Sound Pattern of English. New York: Harper and Row.
Halle, M. & G. Clements 1983. Problem Book in Phonology. Cambridge: MA, MIT Press.