Complex sentence: Icy moons are believed to be a common class of planetoids that have a surface mostly of ice, possibly with an ocean under the ice, and possibly including a rocky core of silicate or metallic rocks.
Split into simpler sentences: Icy moons are believed to be a common class of natural satellites or planetoids with surfaces comprise mostly of ice. It may possibly harbor an ocean underneath it, and possibly include a rocky core of silicate or metallic rocks.