Unbridled means unrestrained. When you find out that you just won the lottery, feel free to jump up and down with unbridled joy. Go ahead, most people would probably let loose in the same way.
A bridle is the contraption used to control a horse, including the reins held by the rider and the bit that goes into the horse’s mouth. Add the un prefix and it means there is no control. Unbridled is often used to describe people’s emotions or actions and it can be either a good or a bad thing. If you have unbridled spending, you’re in danger of going in debt. If you tackle homework with unbridled enthusiasm, you’re likely going to get good grades.