An on-screen display is a very important part of an FPV'ers equipment.
Similar to the HUD in a fighter-plane, the OSD overlays various pieces of critical information onto the video image, which will be viewed by the FPV pilot.
Important questions, such as 'Which way is home', 'What is the state of my battery', are answered at a glance.
Equally important information, such as 'how far away am I', 'how high am I', can help comply with local FPV rules & regulations.
ImmersionRC's approach to on-screen displays is a little different than other manufacturers. Instead of a crowded display, with a bunch of rarely-needed information, just the essentials are shown. FPV is about enjoying the scenery, not admiring the handywork of the OSD manufacturer :-)
The EzOSD is now supported by an iPhone application 'iTelemetry', which allows a plane's position to be tracked in real-time, at the flying field. The simple cable required to interface your video Rx to the iPhone may be purchased here.
This is the first, and currently the only FPV product to be supported on the iPhone!