Complex sentence: The magnetopause is the boundary between a planet's magnetosphere and the plasma solar wind emitted by the sun. It behaves roughly like a droplet of liquid exposed to supersonic flow.
Split into simpler sentences: The magnetopause is the boundary between a magnetosphere and the surrounding plasma. For planets in the solar system the surrounding plasma is the solar wind emitted by the sun. It behaves roughly like a droplet of liquid exposed to supersonic flow.