The Reformer in Pilates is a piece of equipment or machine used to perform Pilates exercises similar to those made on the mat. Doing the exercises on a Reformer machine will challenge your body in different ways compared to, when doing Pilates on a mat. The Reformer is working against the gravity so doing the exercises on the Reformer gives you a sense of elongating. You also work with resistance similar to weights on the Reformer, so the exercises may seem a bit tuffer and may increase your cardio and muscle engagement compared to Pilates performed on the mat.