Nanoporous is like having tiny holes that are too small to see.
Imagine you have a toy bucket full of sand. If you pour water into that bucket, the water will go through the spaces between the sand grains. But if you put a piece of cheese on top of the bucket, the water will not be able to pass through the cheese because the holes in the cheese are too small for the water to fit through.
Nanoporous is like that cheese. It means having tiny holes that are so small, only really tiny things like molecules can get through.
These holes can be made in different materials like metals, ceramics or plastics, and the size of these holes can be controlled by using different methods. Nanoporous materials have many useful properties like being able to separate different molecules or being used for drug delivery in medical treatments.
So, in short, nanoporous materials are like a tiny cheese with super-small holes that can only let in really tiny things like molecules!