What does this mean? Well, not exactly clear at this point. Let´s start by reading an excerpt of what they say in the announcement:
In a sense, inside the Wolfram Language we have a whole computable model of the world. And it becomes trivial to write a program that makes use of the latest stock price, computes the next high tide, generates a street map, shows an image of a type of airplane, or a zillion other things. We’re also getting the free-form natural language of Wolfram|Alpha.
So, we could say the language is a programming language that comes with a huge set of predefined types (and operations on them). Thanks to their knowledge engine, these types represent any existing concept in the world so we can just focus on describing how we would like to combine these concepts in our program instead of explaining the computer what are the attributes and operations we should be able to use on them. Like if you had an almost infinite set of predefined libraries for your language but covering all kinds of real-world objects and not only programming abstractions.
We need to wait to see how this unfolds but so far it looks very promising. It could be the next step up in the abstraction ladder. To know more refer to the original announcement or this other review post and let me know what you think!