The end of each world features a Boss fight. Each boss requires six hits to be defeated. You must hit them three times to unlock the first checkpoint. They will then summon some enemies that you must fight to reach the second checkpoint. You then have to hit the boss three more times to defeat it. Usually, these enemies can be hit when their faces are yellow and not red. Also, if they are on the ground sideways and are yellow, you can simply roll into them with some speed to hit them; no jumping is required. The final Boss last phase may be difficult. Wait until the incoming enemies have been triggered to explode, then start rolling in their direction, jumping over them about halfway through. They should continue onward and hit the Boss themselves.