Milli-Q is a special way of making really clean water that is used in laboratories and other special places where people need very pure water. Imagine you have a big glass of dirty water and you want to make it clean. The first thing you might do is pour it through a strainer to get out the big pieces of dirt or leaves. That's what the first part of the Milli-Q does - it takes regular water and filters out all the big bits of dirt, like minerals and particles.
But even after the big bits are removed, there are still little bits of dirt that you can't see. The second part of the Milli-Q is like adding a special invisible strainer - this gets rid of all the little bits of dirt too, like bacteria or viruses. This is called reverse osmosis, kind of like a magic filter that only lets the clean molecules of water pass through while leaving all the dirt behind.
After the water has gone through both of these steps, it is already pretty clean. But to make it even more pure, the water goes through some more special filters that remove any tiny remaining impurities. This is like the final cleaning step - just like how your mom might wipe down the counter one last time after she cleans up all the big messes.
When the Milli-Q is done, the water is incredibly clean and pure - so pure that it is sometimes called "ultrapure water." This special water is important in laboratories because if there are any little bits of dirt or impurities in the water, it could affect the results of experiments or tests. So, the Milli-Q is really helpful in making sure everything is accurate and reliable.