Opacity is like how easily you can see through something. Imagine looking through a window. If the window is clear, you can see everything on the other side. But if the window is foggy, you can't see through it very well.
Now, think of a material like glass. You might think that you can always see through glass because it's clear. But that's not always the case! Sometimes, even clear glass can stop you from seeing through it. This is because the glass has a measure of opacity.
To describe opacity mathematically, experts use a number called the opacity coefficient. This coefficient tells us how much a certain material blocks light or other forms of energy. The higher the opacity coefficient, the more the material blocks the light energy from being transmitted.
In scientific terms, opacity is measured in units of length divided by mass. This might sound complicated, but it just means that scientists look at how much of the material is there per unit of length - this gives us an idea of how much light energy the material can block.
So to sum it up, opacity is like how much a certain material stops you from seeing through it. Experts use a special number called the opacity coefficient to describe this, which tells us how much light (or other energy) the material can block.