What was the significance of radar during world war ii?

The major significance was it gave the British warning before German bombers even left France to fly over the channel so that fighters could be scrambled to attack them. It also helped direct the fighters once in the air to where the bombers would enter British territory, thereby giving the best chance of shooting them down before they could bomb their targets.

This high loss of bombers was one of the major things that convinced Hitler to end the blitz and the battle of Britain and instead turn on the USSR.