Index-matching material is just like playing with Lego blocks, but for light! Light is like a superpower that travels super fast, and just like how Lego blocks fit perfectly with one another, there are materials that help light waves match and join together. We call those special materials "index-matching" because they perfectly match the way that light travels inside it.
Think of it like this: when light moves from one material to another, it changes direction a little bit. This is because the materials have different properties which bend the light differently. Index-matching material is a special material that is designed to have the same properties as light, so that the light passes through it without bending too much or losing energy.
So when light enters an index-matching material, it is like going through a tunnel of perfect Lego blocks that perfectly fit and join together. This makes the light travel much faster and more efficiently, as it doesn't have to "struggle" to bend and change direction through different materials.
Index-matching materials have many important applications in science and technology, such as in optics, telecommunications, and medical imaging. By using these materials, we can make some pretty cool things!