How different software and apps can talk with each other.
One software will say "hey I got new information" and the other software will reply "got it, thanks!"
Each API has a trigger and an action.
For example, you can trigger an API when someone new signs up for your app (New Row in your Spreadsheet database), and that trigger will cause an action to send them a confirmation email. Or you can trigger an API when someone pays on your website and that trigger will cause all their payment information to be sent to the app (Google Sheet) so the app knows the customer has already paid.
One of the main software tools we use to create no-code APIs is called zapier.com, and there's a high chance you will use it when building your no-code app.
PS- Not all software is open to exchanging information aka... they don't allow API connections. As a first step, make sure the software you want to connect are available on Zapier.com or have an open API