Interoperability
Systems can be designed to interoperate with one another, allowing them to communicate.
We use schemas and protocols to define how to do this, which is implemented on either end.
Modularity is a similar concept, regarding having swappable components in a system.
APIs
API stands for Application Programming Interface, but this name is confusing.
An API is a way to push and pull data from another system.
Your language and framework have an API, which tell you how to write your code that extends the underlying system. https://api.drupal.org is an example.
An external API often provides a service that you can offload from managing within your current system. For example, the Stripe API.
Schema
Systems need to define their data structure so it can be formatted and parsed correctly.
Defining a schema allows developers on different teams to integrate the same way.