ATIV Beam is a new app, exclusive to Samsung's Windows Phone 8 devices, that makes it easy to share pictures, documents, music and videos files with NFC equipped Android devices. While Windows Phone 8 does have support for NFC triggered file transfer it is not compatible with Android for some content types.
Text-based content, such as contacts and notes, generally does work with Windows Phone built in functionality, but multimedia content does not. The ATIV Beam app, by contrast, does work with multimedia files, and can therefore be seen as filling a compatibility hole between Android and Windows Phone devices. It's going to be particularly useful if you're moving to Windows Phone from an Android device, or vice versa.
The ATIV Beam app is similiar to the S Beam feature Samsung includes with many of its Android devices to enable easy file transfer between devices. It uses NFC to trigger a WiFi Direct transfer between two devices (and can also fall back to Bluetooth). On Windows Phone it appears that Bluetooth is being used as the transport mechanism.
Windows Phone Store description:
When this feature is on, you can share pictures, documents, music, and videos by tapping to another NFC-capable android device. Please make sure that NFC sharing status is on in tap+send in settings.
ATIV Beam is a free download from the Windows Phone Store. It is only available for Samung devices running Windows Phone 8 (ATIV S and variants).