The Apple Event Manager allows applications to send and respond to Apple Events. An application that exchanges data with another application will probably use the Apple Event Manager. For example, in a client/server type environment, a client application might send an Apple Event requesting information from a server application. The Apple Event Manager handles addressing and sending the event.
You may see a file called "Apple Event Manager" and wonder what it's for. With AppleScript 1.0, there was a separate extension called "Apple Event Manager 1.0," which fixed some problems with the Apple Event Manager built into System 7. With the latest version of AppleScript (version 1.1) the improved version of Apple Event Manager is part of the AppleScript extension.
For additional information about Apple Events and the Apple Event Manager, see the book "Inside Macintosh: InterApplication Communication."