This is the 5087th most frequent Russian word.
морские
"Морские" means "marine" or "sea-related" in English.
In this sentence, 'морские' is used as an adjective modifying 'волны' (waves), indicating that they are related to the sea.
Морские волны разбиваются о берег.
Sea waves crash against the shore.
Here, 'морские' describes 'звезды' (stars), and refers specifically to starfish, highlighting their aquatic nature.
Мы увидели морские звезды на морском дне.
We saw starfish on the seabed.
In this case, 'морские' is an adjective describing 'животные' (animals), suggesting that these animals inhabit marine, or sea-related, environments.