The medicine ball, which is also known as the Swiss ball, is a giant ball filled with air which is used for the strengthening of your core muscles. While there are a lot of different ways that you can make your core stronger, the medicine ball is among the most interesting, because it forces you to balance yourself in a way that you are most likely not used to doing. Simply laying on the medicine ball and stretching your arms out to your sides is a good way to keep your hip flexors actively engaged, as well as working your stabilizing muscles and your abs to a small extent.
One of the big benefits to using these exercise balls is that they are extremely portable (if you deflate them), and allow you to do a very wide range of different exercises which will work you in a very diverse number of ways. Everything from squats against a wall (with a ball to allow you to roll up and downward) to situps (which force your stabilizing muscles to put in work) all the way up to pushups where either your hands or your feet are up on the ball are great ways to build up your core’s power and endurance.
Of course, the medicine ball is not a substitute for also doing exercises which do not involve it. You will not get huge with a medicine ball, as great as it is for the muscles of your entire body. And while your core will get stronger through its use, keep in mind that definition comes from shedding body fat. Gaining muscle mass and doing cardio are far more effective for getting ripped than the medicine ball.