Undisciplined means uncontrolled or disorderly. An undisciplined bunch of dogs is nearly impossible to walk down the sidewalk together: they all want to run in different directions, chase squirrels, and bark wildly.
When you take the word disciplined, "showing control or order," and add un-, or "not," to it, you get undisciplined. This is a great way to describe an out-of-control classroom, an athlete whose explosive anger gets her kicked off the field, or someone who can't manage his own money well, spending an entire paycheck all in one place.