In the context of a design language, ambience is objective and process.
Ambience as Objective
Ambience is the result of a harmony (fit) between an object and its environment. Like Music for Airports played in an airport. Or reading on a quiet Sunday afternoon. Flow states are, too.
In the design of digital objects, ambience represents a harmony between the digital object, its user, and the environment they both exist within. For the sake of consistency, it may help you to consider these as "archetypal forms": Object, Agent (user), and Environment.
Today, most of our design practices focus primarily on the Object, seeing the Agent through the Object's point-of-view. Likewise, little space is given to the environment except to understand how to design an Object to fit within it ("product-market fit" being the best popular example).
Ambience as Process
We are "products of our environment" says a popular platitude. The universe shaped this planet, the planet shaped us, and we have been shaping ourselves ever since. Language. Writing. Agriculture. Feudalism. Capitalism. Corporations. Democracies. Many of these inventions have become so enmeshed in our daily lives as to become ambient (i.e. invisible). You don't think about the years of schooling that taught you to read these words, or the effects that the English language has on how you think, reason, and engage with the world. You just do it.
These systems have far more influence over our designs than we do. Our brains are a cognitive system that enabled the development of language. Agriculture created "ownership", calendars and new coordination technologies. Today's power-law based VC model incentivizes the creation of very specific types of technology companies that produce a very specific type of digital product.
In APD, design is practiced in the context of these influences: of what we can, and cannot control. It helps us understand where these influences are coming from, how they will naturally sabotage or support our efforts, and how we can use them to design towards an ambient, harmonious outcome.
Ambience as influence
At a certain point, effectively abstracted infrastructure like global shipping, tap water and the internet become ambient - harmonious and invisible - as the output energy necessary to use them continues to fall below a noticeable threshold.
Additionally, various larger systems become so well established in social groups as to become ambient and invisible to their participants, without actively looking. Culture and customs. Money. Capitalism. Consumer behaviors. Etc.
And of course there are natural systems that govern how we have evolved and how we continue to think and act in the world, which cannot be perceived without formalized processes like The Scientific Method, or languages like mathematics.