In-flight entertainment (IFE) is the system of seat-back screens in an airliner's cabin, and the onboard servers that stream films, music and maps to them. It is the screen in front of you on a long flight.
Take, for example, a flight from Delhi to London with 250 passengers. Before the flight, the airline's team loads the films, shows and music onto a server in the aircraft. The server sits in a rack under the cabin floor. A passenger taps a film on the screen. The server sends the film down the cabin's network, to that seat only. Many passengers can watch different films at once, because the server streams a separate copy to each screen.
The airline does not own the films. It pays film studios and music companies for the right to show them, and the content is usually locked so that only the aircraft's system can play it. The airline updates the content every few weeks, so that regular flyers see new films.
The system also shows the moving map. The map needs the aircraft's position, speed and height. These come from the aircraft's avionics. So IFE must be connected to avionics, but safely: it can only read the flight data and must never send commands to the flight systems. This is why it is certified under safety certification rules as part of the cabin.
Some aircraft also have Wi-Fi in the cabin. In-flight Wi-Fi is a different system. It connects the cabin to the internet through satellites, so that passengers can browse and message. IFE streams from the aircraft's own server and needs no internet. A newer approach streams films to the passenger's own phone or tablet instead of a seat-back screen.
The aircraft maker fits the IFE during final assembly of the cabin, and the airline decides which content to load. Seat-back screens are heavy and each one adds weight, so some airlines now prefer to stream to phones.