Psamment is basically just a fancy word for sand that is found in certain types of soil. You know how when you go to the beach and you see all of the little grains of sand? Psamment is kind of like that, but it's not always found at the beach. It can also be found in sandy soil that is used to grow different types of plants.
Now, why is psamment important? Well, it's because it can affect how well plants can grow in certain areas. You see, plants need water and nutrients from the soil to survive. If the soil is too sandy (like with psamment), then it won't hold onto water very well. This means that it might not have enough water to give to the plants. On the other hand, if the soil has too much clay (which is another type of soil), then it might hold onto too much water and drown the plants.
So, if you wanted to grow something like a vegetable garden, you would want to make sure that the soil you're using has just the right amount of psamment (not too much, not too little) so that your plants can grow nice and healthy. It's just one of many things that people have to think about when they're trying to grow plants!