If you are getting braces or an appliance to support your orthodontic treatment such as a TPA, there is a good change you will have bands on your back teeth rather than brackets. Bands are, as they sound, thin metal bands that are cemented to your teeth to provide support for an appliance and to engage the arch wire.
Your orthodontist is unlikely to fit your bands without some preparation first as you will likely find your back teeth quite close together and not offering sufficient space between the teeth to get the bands placed. This is where spacers are used to create the required gap between your teeth. Spacers are small pieces of elastic that are stretched between your teeth and when in place, gently exert pressure to create a small gap which is large enough for the band to be fitted.
I remember getting spacers the first time I had treatment and although not uncomfortable, it feels like you have a piece of food stuck between your teeth that you cannot remove. They can also make your teeth quite tender so eating soft food might be advisable for a few days after getting the spacers fitted. Spacers are usually left in for a couple of weeks to work their magic so that when you attend for your appliance or braces fitting, your orthodontist will have created sufficient room to accurately seat the band on your tooth.