Hyaluronic acid, or a similar gel-like substance, is the main component of most fillers. Your body already produces hyaluronic acid to hold moisture and keep your skin hydrated. When used in lip injections, hyaluronic acid or imitation molecules attach to water molecules in your lips to keep them supple and rounded.
The substance is also easy for your doctor to control and shape, so you have a high chance of getting your desired look.