A specification language is a way of making sure that different manufacturers of a product make it the same way. It is kind of like a recipe for how to make something.
For example, if you wanted to make the same kind of shoes, you would need to make sure that all of the materials, designs, and sizes are the same. The only way to make sure that all manufacturers make the exact same kind of shoes is to write down exactly what materials and sizes should be used and how they should be put together in a language. This language is called a specification language.
When manufacturers follow the same specification, customers can be sure that they will be getting the same exact product each time they buy it - just like the same shoes will look the same and fit the same, no matter the manufacturer.