A repeater is an device that receives radio transmissions on one frequency, and simultaneously re-transmits them on a second frequency with more power.
Nearby walkie-talkie or other users whose radios are set to use these two frequencies can then communicate via the repeater.
Its purpose is to increase the area of signal coverage of a group of walkie-talkie or other radio users.
The walkie-talkies are set to transmit on one frequency, which is picked up by the repeater, and then instantly re-broadcast on a second frequency.
The walkie-talkies must be set up to work with a repeater.
The repeater is an electronic "box of tricks" with an aerial attached.
It needs mains power, and it needs to be out of the weather (although the aerial can obviously be outside).
The repeater (or at least it's aerial) should be positioned somewhere near the centre of the area you want to cover, and as high up and in as unobstructed position as is possible.
A repeater can only "repeat" one frequency at a time, so if your event uses several radio frequencies for different purposes you would need more than one repeater.