What does it mean if my baby is shivering?

Depends: During the first weeks of life, baby's nervous system is immature. Babies can shiver or shake related to that immaturity. In an older baby, shivering often precedes fever, or happens when temperature is rising quickly.

Newborn response: The obvious answer is that they are cold, but if they are very young, under 3-4 months, they are likely having a moro reflex, a primitive muscle reflex where moving them a little bit or jostling them causes the arms, legs, or jaw to "quiver" as if they are shivering. Older children shiver in an effort to raise temperature, such as when generating a fever, so if they are shivering, monitor temp.