Teeth normally fit tightly against one another. Spacers are inserted before placement of your braces to provide some space between teeth for attaching the bands. There are two types of spacers, small springs or plastic modules. In just a few days they gently move desired teeth slightly apart.
Spacers often cause some soreness, but this goes away in a few days. Rinse your mouth with warm salt water to relieve the irritation. Your normal chewing also helps to get your mouth feeling better.