Low-iron glass is a special kind of glass that doesn't have as much iron in it as normal glass. Normal glass looks a little bit green, especially around the edges, because it has small amount of iron in it. But low-iron glass is very clear and looks more like invisible. It's like the difference between water you drink and water you swim in a swimming pool.
Why is it important to have low-iron glass? Well, there are many reasons. If you want to put up a big window, you might want it to be as clear as possible without any green tint. If you are building a greenhouse, low-iron glass will let more sunlight through and make the plants grow better. If you are making solar panels, low-iron glass helps the panels absorb more energy from the sun.
So, low-iron glass is just like normal glass, but it doesn't have iron in it so it's very clear and can do some special things that normal glass can't do.