RWR stands for Radar Warning Receiver. You will hear tones (at least in real life) when a radar beam passes over your aircraft. The tone will become more intense when you are locked. By monitoring the sources of radar beams hitting your aircraft, the system is able to tell you the bearing and threat level of each return. It does not measure distances however. So when you look at the RWR, and you see a blip that is closer to you than another, it does not mean that emitter is closer to you, it means that he is a greater threat.
The RWR monitors each return for signs of a launch, generally the lock/activation of a radar-guided missile, ARH or SARH these days. Because these missiles (and the aircraft firing them) send out special signals when they are launched, the RWR will let you know. However, if the missile is a passive seeker, infra-red like the sidewinder, or Electro-optical like assorted SAM loads, you will receive no warning. It does not send anything out, just takes things in, like your heat signature, or visible shape. So they're extra dangerous, but also easier to fool, if you notice them at all.
That's the danger of a MANPAD like the SA-5 (I believe is used in WOE, correct?), hand-launched and with no warning, it's very easy to miss.