This is a fancy term, but just means that it's a substrate that comes packed with nutrients perfect for your new plants.
While it is very possible to grow aquatic plants in plain sand or gravel, I think you are better off as a new fishkeeper spending a few more bucks and purchasing what's called an active nutrient substrate. This is a fancy term, but just means that it's a substrate that comes packed with nutrients perfect for your new plants. Plain gravel and sand is relatively inert and contains nothing that our plants need to grow. Rather than waiting several weeks or months for that material to accumulate nutrients from fish waste, why not hit the ground running and get a leg up. Add enough substrate to cover the bottom with roughly 1-3 inches and try sloping it from front to back.