The Notion API (Application Programming Interface) allows developers to connect Notion with external apps and services. This means that developers can use Notion data in other apps, automate tasks and create custom integrations.

For example, using the Notion API, developers can:

1. Pull data from a Notion database and use it to create charts and visualizations in another app.
2. Create a custom integration that automatically saves new entries from a form on a website to a Notion database.
3. Automate the process of creating new pages in a Notion project by triggering it with an external event, like a button press.

Using the Notion API requires a basic understanding of web development and programming. Notion provides detailed documentation and examples to help developers get started with the API. As with Zapier, the use of the Notion API also requires a paid subscription to Notion.

It's worth noting that Notion API has a usage limit, so it's important to be mindful of the number of requests made to the API and to cache data to minimize the number of requests.