External API is an interesting api that allows communication between JavaScript and the flash player. This is a very important feature for sites that have flash content in conjugation with JS code.

ExternalInterface is a part of this api that specifically concentrates on easy communication between ActionScript and the Flash Player.

As the Adobe site mentions you can do the following on the HTML page:

• Call any JavaScript function.
• Pass any number of arguments, with any names.
• Pass various data types (Boolean, Number, String, and so on).
• Receive a return value from the JavaScript function.

From JavaScript on the HTML page, you can:

• Call an ActionScript function.
• Pass arguments using standard function call notation.
• Return a value to the JavaScript function.

Here is the tutorial on the Adobe site explaining how External API works.
This api was introduced as part of Flash Player 8.