When you toddle, you wobble a bit on your legs as you walk. It's most common to see very small children toddle around.
A baby who's just learned how to walk can be said to toddle, staggering a little on short legs. The littlest kids at preschool still tend to toddle around their classroom, somewhat unsteady on their feet. The verb toddle comes from a Scottish word whose origin isn't known — the earliest meaning was "to play." The noun toddler comes from toddle, and it means "child just learning to walk."