Okay kiddo, so imagine you have a balloon and you blow air into it until it's really big and round. Now imagine if there were tiny little holes all over the balloon and some of the air you blew in leaked out through those holes.
That's kind of what happens with hydrogen gas in materials like metal. When metal is melted down and then solidified, tiny bubbles of hydrogen gas can get trapped inside. These bubbles are called porosity.
Porosity can weaken the metal and make it more likely to break or crack. Think of it like putting air bubbles in your playdough. If you have too many bubbles, it's not as strong and can fall apart easier.
So when people talk about hydrogen gas porosity, they're talking about those little bubbles of gas that get trapped inside metal or other materials. It's important to try and prevent this from happening so the material stays strong and doesn't break easily.