A repeater is an device that receives radio transmissions on one frequency, and then simultaneously re-transmits ("repeats", hence the name!) them on a second frequency, but with more power because it can draw on mains electrical power and usually a bigger aerial than on a small battery-powered walkie-talkie radio.
Nearby walkie-talkies or other users whose radios are set to use these two frequencies can then communicate via the repeater.
The purpose of the repeater 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 is about the size and weight of a large desktop PC's "system unit".
It needs mains power, and it needs to be out of the weather (although its aerial can obviously be outside).
The repeater (or at least it's aerial) should ideally be positioned somewhere near the centre of the area you want to cover, and as high up and in as unobstructed a 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.
Walkie-talkies need to be set up to use a repeater on one or more of their channels. You cannot simply get a repeater and assume it will work with some existing radios.
A walkie-talkie's channel that is set for use without a repeater, will not work via a repeater.
Fortunately, we have set up all of our "hire fleet" of walkie-talkies so that 4 of their 16 channels are ready to work with our repeaters for hire, so they are always "ready to go" for use with or without one of our repeaters without any need for reprogramming.