Ballos is a boss in a video game called Cave Story. He is a big, scary monster who lives deep within a cave. To beat him, the player has to fight him using weapons and dodging his attacks. It's like playing a game of "tag" or "hide and seek", but with more danger and excitement. When the player finally beats Ballos, they feel a huge sense of accomplishment and pride. It's kind of like when you climb to the top of a big slide and feel really happy and proud of yourself for doing it!