"Original intent" means trying to figure out what the people who wrote or made a law meant when they wrote it. It's like trying to understand what your mom meant when she told you to clean your room. Did she mean just put your toys away, or also pick up your clothes?
When some people are deciding how to interpret a law or rule, they will try to think like the people who made it. They will look at the words and try to figure out what they meant when they wrote them. For example, if a rule says "no loud music after 9 pm," the people trying to follow the rule would think that the original intent was for people to be quiet at night and not disturb others who are trying to sleep.
Sometimes people disagree about what the original intent of a law or rule was. Just like how you might argue with your friends over what your mom meant when she said to clean your room. But by trying to understand the original intent, people hope to make sure they are following the rules properly and treating others fairly.