A cartouche is a piece of parchment paper cut to fit inside a pan and laid over or close to its contents. It limits surface drying and evaporation while letting some steam escape, making it useful for poaching fruit, simmering vegetables and braising.
It behaves differently from a tight-fitting lid. Use heat-safe cooking parchment rather than wax paper, and keep loose edges away from flames or heating elements.
Reference: Further reading.