What does it mean to maintain a stable internal environment?

It means to guide and regulate chemical and physical parameters to keep your body or environment in a constate state for proper functioning.

Explanation: This is called homeostasis. The body has systems in place to monitor its components such as cells, organs and any other molecule needed to maintain life and good health. The body needs to maintain its temperature at 37°C, pH of blood at 7.4, water balance, osmotic pressure, growth rate and tissue repairs whenever it dictates the need, danger of infection or injury.

All of these form an integral part of a healthy living organism and it needs to always be stable, regulated and monitored. This is called maintaining homeostasis.