Radar warning receiver
A Radar Warning Receiver (RWR) is a system that informs the pilot about incoming radar radiation. In game the RWR is a clock display that shows the type of threat (detected by radar or tracked by radar) and its direction.
If a radar is sending a single "ping" at you (a search radar is detecting you), the RWR will sound a short warning and show the direction it is coming from. RWR can show multiple pings at once from different directions and sources. If you hear a constant alarm and see a striped line coming from the ping towards the center of the RWR, a tracking radar has locked you and is constantly tracking your position (which usually precedes a missile launch).
Some helicopter RWR sets in-game have what's known as MAW (Missile Approach Warning), which indicates a missile launch that's heading directly to the user's vehicle. A few of these helicopters have the ability to automatically drop countermeasures when a MAW warning is active.
In Update "La Royale", soviet aircraft got a slightly different RWR with non-precise sectors which indicate the approximate location of the ping or lock.