Simplify the following text for a Grade 9 native English-speaking person.
Original: Lagoon refers to both coastal lagoons formed by the build-up of sandbanks or reefs along shallow coastal waters , and the lagoons in atolls , formed by the growth of coral reefs on slowly sinking central islands .
Simplified: Lagoon refers both to coastal lagoons , and the lagoons in atolls , formed by the growth of coral reefs on slowly sinking central islands .