API stands for Application Programming Interface. It's a ruleset that defines how programs communicate with each other.
In the construction context, any software or platform (and even digital tools!) we use to do our work has an API. Most of the time, we don't see the API at all. Programmers use the API to build a user interface for us less technical users.
Because of the nature of our specialized work, design software usually has an accessible API. We leverage these APIs to build integrations and automations that fit the work we do. It's impossible for a software provider to consider every use case, so there is a robust ecosystem of software extension filled with domain specialists.