Muscle afferents are nerves that convey stretch information from muscle spindles to the spinal cord. Muscle spindles are essentially tiny muscle fibers, interspersed in parallel among regular muscle fibers, that provide a kind of proprioception, telling the CNS how long your muscles are and how rapidly they are changing in length. You'll learn all of this in medical school; I'm not sure it's fair game for the MCAT.
"Afferent" means that the information is Approaching the CNS; "efferent" means that the information is Exiting the CNS.